Methods for optimizing the performance, cost and constellation design of satellites for full and partial earth coverage

ABSTRACT

A system and method for highly efficient constellations of satellites which give single, double, . . . k-fold redundant full earth imaging coverage, or k-fold coverage for latitudes greater than any selected latitude is given for remote sensing instruments in short periods of time, i.e., continuous coverage, as a function of the parameters of the satellite and the remote sensing instrument for many different types of orbits. A high data rate satellite communication system and method for use with small, mobile cell phone receiving and transmitting stations is also provided. Satellite instrument performance models, full and partial satellite constellation models, and satellite cost models are disclosed and used to optimize the design of satellite systems with vastly improved performance and lower cost over current major satellite systems.

RELATED APPLICATIONS

This is a U.S. Utility Application which claims the benefit of U.S.Provisional Application No. 61/153,934, filed Feb. 19, 2009, the entirecontents of which are incorporated herein by reference.

BACKGROUND

Satellites are used for remote measurements of the earth in a widevariety of areas. These include remote measurements of the earth'satmosphere, its land surface, its oceans, measurements for defense,intelligence, and communications. In each field, various instruments areused to make active and passive measurements. For atmosphericmeasurements, examples are the remote profiling of temperature, theprofiling of trace species, measurements of the wind field andmeasurements of the properties of clouds and aerosols. Thus, satelliteremote sensing technology and methodologies may be used to perform avast array of different measurements. In addition, satellite remotesensing has also been used for planetary measurements.

Measurements from low earth orbit satellites with altitudes in the rangeof 200 to 800 km provide measurements with high spatial resolution butwith only relatively infrequent full earth coverage, for example, onceper ½ day to a few days. On the other hand, measurements from satellitesin a geosynchronous (GS) orbit provide frequent coverage, of the orderof seconds to hours. Typically, a GS orbit is a very high altitude orbitat about 36,000 km altitude above the equator with the property that thesatellite has a rotational period around the equator equal to therotational period of the earth. Thus, a GS satellite stays positionedabove the same point on the equator. This allows measurements from thisplatform to view a large portion of one side of the earth with frequentcoverage.

The resolution of measurements from a satellite is limited by theeffects of the earth's curvature, the scan angle of the instrument, andthe altitude of the orbit. For a GS satellite, measurements made inareas at high latitudes have degraded resolution which for latitudesgreater than 60° is significant. If the effective measurement area for asingle satellite corresponds approximately to the range of latitudesfrom −60° to +60°, and, similarly, the range of angles in the equatorialplane from −60° to +60°, it follows that at least 3 GS satellites arerequired to cover the area in the plane around the equator. GSsatellites do not, however, allow reasonable resolution to be obtainedin areas above 60⁰ latitude. Moreover, GS satellites do not allow anycoverage in the Polar Regions, since a view of this area is blocked bythe curvature of the earth and the approximate 8.6° angle of the polesas seen from a GS satellite. For 3 GS satellites, the net angle ofmeasurements of the earth at the edge of the scan in the equatorialplane or at ±60° latitude is approximately 68°.

High altitude GS or stationary satellites in equatorial orbits at about36,000 km altitude collect earth imaging and communication data forvarious different user communities. The data include the ultraviolet,visible, near infrared, infrared, microwave and radar frequency regions.The data collected are often in the form of digital imaging data whichcan be used to make pictures or imagery in one or potentially thousandsof spectral regions for photographic or computer aided analysis. Thedata can be collected in the frame time, a short period of time of theorder of seconds to hours, with about 3 satellites from about −60° S to+60° N latitude. Close to full earth coverage can be obtained with anadditional 3 to 6 high altitude satellites. The data obtained are lowresolution, however, even for very large expensive systems because ofthe very high altitude orbits.

LeCompte, in a series of six patents and patent applications (U.S.Patent Application Nos. US 2003/0095181A1, May 22, 2003; US2002/0089588, Jul. 11, 2002; US 2002/0041328, Apr. 11, 2002; U.S. Pat.No. 6,271,877 issued Aug. 7, 2001, U.S. Pat. No. 6,331,870 issued Dec.18, 2001; and U.S. Pat. No. 6,504,570, issued Jan. 7, 2003) discloses asystem for measurements from a Geostationtionary, Geosynchronous orbit.He provides a system, methods, and apparatus for collecting anddistributing real time, high resolution images of the earth with asensor based on multi-megapixel CCD arrays. The system utilizes at leastfour, 3 axis stabilized satellites in Geostationary orbit to provideworld-wide coverage excluding the poles. The current disclosure usesnon-geosynchronous orbits at various altitudes to provide measurementswith much higher performance and resolution, full earth coverageincluding the polar regions and areas at high latitudes, and significantcost advantages over prior art systems.

The use of a zoom type feature, which is similar to the zoom feature ofa camera, has been used on geosynchronous GS platforms. This featureallows a small area of the earth to be observed with more detail thanwould normally be the case. However, in order to obtain this detailedview, the coverage of the rest of the earth that would normally beviewed by a given satellite is lost. For a one satellite GS system, thiswould result in a loss of all of the data that would normally beobtained, and in a 3 satellite GS system this would result in a loss of⅓ of the data, which severely limits the use of this feature.

For a low or mid-altitude satellite (LMAS) constellation, an ultra-highperformance measurement can be made in any given small area of the earthby acquiring and viewing only that area when it first appears in aportion of the area being scanned by a given satellite. The coverage ofthe small area continues until the satellite leaves the coverage areaand as coverage of the small area of interest from a given satelliteends, coverage of the small area of interest from the followingsatellite begins. This process provides continuous coverage of the smallarea of interest from successive satellites. For a system with nsatellites, up to n ultra-high performance measurements of small areascan be made simultaneously, one measurement for each satellite.

The only loss of coverage as a result of the use of the ultra-highperformance feature is in the single area containing the small featureof interest. For an LMAS system with 200 satellites, this would resultin a loss of coverage of 1/200 or 0.5% of the coverage of the earth.Moreover, an LMAS system has a lower altitude than a GS system, whichadvantageously allows a much higher diffraction limited resolution thana GS system, e.g., more than 100 times higher resolution for a 200 kmaltitude LMAS system.

Luders (1961) described constellations for continuous, complete globalcoverage using computer search methods with the “streets of coverage”technique for polar and inclined orbit constellations. Rider (1985) andAdams and Rider (1987) described continuous global coverage for single,double, . . . k-fold redundant coverage using the “streets of coverage”technique for optimal, i.e., the minimum number of satellites for agiven coverage, polar orbit constellations. They also described k-foldredundant coverage for latitudes above given latitudinal planes, i.e.,0°, 30°, 45°, and 60°. Rider (1986) used the “streets of coverage”technique to obtain constellations for inclined orbits. Walker (1970)used circular polar orbits and, in 1977, used inclined orbits to obtainorbit constellations using computer search and analytic techniques whereeach satellite of the constellation can have its own orbital plane. Thiswork was used only for the case of very small satellite constellations.

The “streets of coverage” technique uses a conical scan pattern to scanthe area within a series of minor circles which are centered at eachsub-satellite point on the spherical surface of the earth. The area ofcontinuous overlay of these circular patterns on the earth at a givenpoint in time for satellites in one orbital plane and for satellites indifferent orbital planes then describes the area of continuous coverage.For continuous whole earth coverage, Rider gives a complex analyticmethod for determining the orbital planes, the number of satellites, theeffective “streets of coverage” angle of the conical scan pattern, andthe multiplicity of the redundant, i.e., single, double, or k-foldcoverage as a function of these parameters. Results are given foroptimally phased and unphased polar satellite constellations forconstellation sizes up to approximately 160 satellites for singlecoverage.

Patent WO 03/040653A1, filed on Nov. 11, 2002 by A. B. Burns, “ImprovedReal or Near Real Time Earth Imaging Information System and Method forProviding Imaging Information” claims to provide methods for contiguousor overlapping coverage over the earth (95% of earth) for imagingmeasurements from relatively low altitude elliptical polar orbitingsatellites at 640 km, as well as for other non-satellite platforms. Onpages 52-55 of this patent, “the specific configuration of the satellitenetwork having particular regard to how real time global coverage isachieved” (p. 52), is described. The method, calculations, andinstructions the patent gives for obtaining contiguous, overlappingcoverage “so that the footprints contiguously and concurrently cover asubstantial part of the earth's surface continuously and dynamically”(p. 4), for 95% of the earth's surface are as follows. First, thesurface area of the 95% of the earth covered is calculated as 4.856*10⁸km² (p. 53, lines 11 and 12). The effective area covered by a singlesatellite is then calculated as 212,677.58 km² (p. 54, line 21). “Thenumber of satellites required to image a given proportion of the earth'ssurface” (p. 55, lines 1-2) is then given as

$\begin{matrix}\begin{matrix}{{{Number}\mspace{14mu} {of}\mspace{14mu} {satellites}} = {{area}\mspace{14mu} {to}\mspace{14mu} {be}\mspace{14mu} {covered}\text{/}{area}\mspace{14mu} {of}\mspace{14mu} {coverage}}} \\{= {4.856*{10^{8}/212677.58}}} \\{= {2283\mspace{14mu} {{satellites}.}}}\end{matrix} & (A)\end{matrix}$

Equation (A) requires that each satellite in the constellation ofsatellites cover a different area on the earth of the same size, 212,677km². That is, the satellites must provide uniform spatial coverage overthe earth. FIG. 31 b in the Burns patent shows a series of thesecontiguous coverage areas of equal size. However, Eq. (A) and the Burnspatent do not give a correct method for how this uniform coverage couldbe achieved for the case of satellites.

For example, FIG. 28 is one of the four detailed figures for imaging inthe Burns patent. It is described in the brief description of thedrawings as “a diagram showing how a single polar orbiting satelliteimages the earth's surface”. It shows a series of parallel orbits goingin a north-south direction which have approximately uniform width in theeast-west direction and appear to give uniform coverage. These orbits,however, do not go over the poles (with the exception of the centralorbit) and are thus not the required polar orbits as specified in thepatent. These orbits also do not go around a circumference of the earth(with the exceptions of the central orbit), that is, they do not makegreat circles or ellipses around the earth, and therefore, are notpossible satellite orbits. Thus, this figure does not describe satelliteorbits except for the central orbit and cannot provide the uniformcoverage specified in the patent.

The Burns patent also states that elliptical polar orbits are requiredand that circular polar orbits will not work. Burns, however, onlyattempts to treat the case of a circular polar orbit as described by Eq.(A) with an orbit altitude of 640 km and does not attempt to treat thecase of an elliptical orbit.

Polar orbiting satellites have the property that the satellite orbitpasses over the north and south poles. As shown in FIG. 1 of his patent,and similar figures in other patents, constellations of polar orbitshave maximum orbital separation at the equator, the orbits converge andthe separation decreases at mid-latitudes, and the satellite orbitsconverge and essentially totally overlap as the orbits approach thepoles. As a result, the satellite spacing is maximum at the equator inthe longitudinal, east-west, direction, decreases significantly atmid-latitudes, and goes to zero at the poles. The corresponding coverageper satellite is highly non-uniform with the amount of coverage andoverlap varying by more than 20 times over the surface of the earth forthe case Burns considers.

As discussed above, Eq. (A) assumes uniform satellite coverage over theearth to calculate the number of satellites. Since this does not occurfor polar orbiting satellites, Eq. (A) and the Burns method isfundamentally incorrect. Further, if Eq. (A) is used, then overlap incoverage in one area of the earth, e.g., as occurs in the longitudinaldirection at high latitudes for polar orbits, must be compensated for bycorresponding large gaps in coverage in other areas of the earth. Thesegaps in coverage result from the Burns methodology and do not allow thecontiguous/overlapping claims of the Burns patent to be realized forsatellites.

The equations in this disclosure can be applied to the Burns patentparameters to calculate the number of satellites required for full earthcoverage. The chord length of his measurement 2X is determined from thesquare root of Burns' effective area of coverage, 212,677.6 km² for asingle satellite, which is a square, and which yields 2X=461.17 km. FromEqs. (1) and (2), the number of polar planes required as the satellitespass through the equatorial plane are calculated as n_(e)=44, and therequired number of satellites per polar plane from Eq. (4) as n_(p)=87.This, in turn, gives the minimum number of satellites needed forcoverage over the earth from Eq. (6) as 3828. This calculation includesthe effects of overlapping coverage.

The 3828 satellites required based upon the foregoing are considerablylarger than the 2283 satellites determined by Burns using his Eq. (A).To get full earth coverage in the polar plane with the Burns parameters,87 satellites per polar plane are required. When the total number ofsatellites calculated in the Burns patent is divided by 87 satellitesper polar plane, we find that only 26.24 planes would be availableacross the equatorial plane. This would provide only approximately 60%coverage and would give 40% gaps in coverage in the equatorial plane,i.e., 26.24/44≈0.6. A similar result is obtained for the percentcoverage and gaps in coverage in the polar plane.

Thus, it is clear from the preceding detailed calculation that tocompensate for the overlapping coverage inherent in constellations ofpolar orbiting satellites, the Burns patent methodology produces largegaps (approximately 40%) in full earth coverage. Since his stated designwas for 95% full earth coverage, the effective gap in coverage in theBurns methodology is about 35%. Thus the contiguous/overlapping claimsof the Burns patent cannot be met for satellite application.

SUMMARY OF THE INVENTION

The basic method that is used in the current disclosure for thesatellite constellation design for the case of polar orbiting satellitesis outlined as follows. The method uses a scan over two perpendicularangular planes, one in the plane of the satellite motion, the other inthe cross-track direction, which is more efficient than and in contrastto the “streets of coverage” technique, which uses a conical scan toproduce circular scan patterns on the earth. The method also uses aphysically based criterion to determine the minimum number of satellitesrequired to produce continuous, complete earth coverage.

For polar orbiting satellites, the equatorial plane has the greatestcircumference of any latitudinal plane. To provide contiguousoverlapping coverage over the earth, contiguous or overlapping coverageover the equatorial plane must first be established. Once this isachieved, full coverage over every other latitudinal plane will exist.In fact, overlapping coverage over every other latitudinal plane willalso exist.

The methodology of the constellation design of the present invention isfounded upon an analytic technique. First the angular separation of theconstellation of satellites in the equatorial plane (as they passthrough this plane) is calculated from Eq. (1), followed by acalculation of the number of measurements needed in the equatorial planefrom Eq. (2). This is rounded up to get the number of polar planesn_(e), or equivalently, the number of satellites needed as they crossthe equatorial plane. Then the satellite angular separation in theequatorial plane is recalculated from Eq. (3), and the number ofsatellites n_(p) needed in each polar plane based on Eq. (4) is thendetermined. The satellite angular separation in the polar plane isderived from Eq. (5), and the number of satellites needed for full earthcoverage is calculated using Eq. (6).

The constellations presented herein are generally much more efficientfor continuous single, double, . . . k-fold redundant global coveragethan prior art constellations. Namely, the same coverage can be obtainedwith the same altitude for constellations using the methods of theinstant invention with about 40% fewer satellites than those required byRider. This applies for constellation sizes of 8 or more satellites.

This disclosure describes innovations in at least three areas which arecombined to produce new satellite system designs and measurementcapabilities with much higher performance than the current state-of-theart. These areas are as follows. One—performance models are developedwhich give the performance of a wide variety of satellite instruments interms of the satellite, instrument, and constellation parameters.Two—highly efficient constellation designs and methods are developed forfull and partial satellite constellations. Three—cost models aredeveloped which relate the instrument performance parameters, theconstellation parameters, and the system cost. The combination of thesethree methods allows the satellite system performance to be optimizedfor any given cost. Using the combination of these methods, it is shownthat full satellite constellations with simultaneous full earth coveragecan be used to obtain much higher resolution than geosynchronous andhigh altitude systems. It is also shown that partial satelliteconstellation designs can be used to obtain greatly improved satelliterevisit time or greatly improved resolution over current low altitudesatellites. It is also shown that high performance low altitude systemscan be used to achieve continuous coverage. Further, these systems havethe same or lower cost as current low performance systems.

Methods are provided herein for the design and configuration ofconstellations of satellites which give single, double, triple, . . .k-fold redundant full earth imaging coverage for remote sensinginstruments in short periods of time, i.e., essentially continuouscoverage. Methods are also disclosed for the design of constellations ofsatellites which give single, double, triple, . . . k-fold redundantcoverage for all latitudes greater than any selected latitude. Theconstellation design is given for polar orbiting satellites as afunction of the altitude of the orbit and as a function of theparameters of the remote sensing instrument as well as for a number ofother different types of orbits.

These methods are also used to provide the design for high data ratesatellite communication systems for use with small stationary or mobilecell phone stations. These systems use low altitudes with small zenithangles and as a result have large signal advantages over currentconventional geosynchronous or low altitude systems.

General methods are presented for evaluating and comparing theperformance of remote sensing instruments for different satellite,instrument and constellation parameters for active and passiveinstruments. The methods demonstrate that full earth coveragemeasurements with small zenith measurement angles can be obtained withlow or mid-altitude satellite LMAS constellations with much higherperformance than can be obtained with GS satellite systems, in times asshort as a fraction of a second. This requires constellations with alarge number of satellites.

Methods are also given which allow trade-offs between the performance ofremote sensing systems, the orbital altitude, the zenith measurementangle, the constellation size and cost. It is shown that constellationsof low or mid altitude satellites with small telescope diameters havethe same or lower cost as one or a few large satellites. The resultantLMAS systems have 25 times higher resolution than GS systems, the sameor lower cost, fast measurement times, and full vs. partial earthcoverage. These LMAS systems can thus replace GS systems for mostapplications. The cases of both signal limited and diffraction limitedperformance are given.

Methods are also presented for the design of partial constellations ofsatellites that greatly reduce the number of satellites required forfull earth coverage compared to full LMAS constellations. These partialconstellations give full earth coverage in a time of a fraction of asatellite orbit for one design and in times of several orbits for asecond design. This has particular application to low altitude systemsas well as to GS systems.

General methods are given which allow performance, cost, and partialconstellation models to be used to improve the satellite revisit timefor high performance, low altitude satellites. It is shown that thesatellite revisit time can be reduced by more than 100 times fromapproximately 12 hours to 2 minutes at the same cost and with the sameperformance. These methods can be used to replace single low altitudesatellites with LMAS systems which give much more frequent coverage or,alternatively, higher spatial resolution. Methods are also given whichallow continuous coverage to be obtained. The case of both signallimited and diffraction limited performance are presented.

Methods are also given that use an ultra-high performance mode forimaging small spatial areas of interest with essentially continuouscoverage and very high performance.

The methodologies given in this disclosure apply to both 2D and 3D,stereo, measurements of the earth with multiple redundant, e.g., 2-, 3-,or k-fold continuous, full earth coverage. The case for 3D coverage istreated for both stationary and high velocity moving objects.

The methods herein defined apply to a large number of different types ofsatellite constellations and circular and elliptical orbits includingpolar orbits, polar orbits rotated by an angle φ relative to the polaraxis, equatorial orbits, and orbits making an angle γ with theequatorial plane.

The methods given in this disclosure for improving the performance andcost of satellite systems can be used with the constellation designsgiven in this disclosure. These offer significant advantages over priorconstellation designs. Alternatively, the methods given here forimproving the performance and cost of satellite systems can also be usedwith other constellation designs.

The LMAS constellations use small diameter telescopes and thus use lowlevel technology. The LMAS constellations also have a large number ofsatellites and are thus insensitive to the failure or loss of a singlesatellite

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1—Geometry of a scanning measurement for a satellite at altitude zwith scan angle θ, range R, arc length arc, sag height z₂ and, as seenfrom the center of the earth, a scan angle θ′, chord length 2X, andz₁=R_(o)−z₂ where R₀ is the radius of the earth. It is noted that z=z,z₁=z₁ and z₂=z₂ are used. interchangeably.

FIG. 2—Cross track scan angle θ_(x) and along track scan angle θ_(y) inthe same plane as satellite velocity vector v.

FIG. 3 a—cross track scan with linear detector array (LDA).

FIG. 3 b—pushbroom scan using the satellite motion for the along trackscan with a linear detector array LDA.

FIG. 3 c—multiple position 2D step scan (SCi) with 8 positions.

FIG. 4—Projection of the satellite track on the earth for a polarorbiting satellite.

FIG. 5—Contiguous/overlapping areas of size θ_(x) by θ_(y) seen by aconstellation of satellites scanning a small region near the equator(for small angles θ′_(x) and θ′_(y) as seen from the center of theearth).

FIG. 6—Four polar satellites scanning slightly more than 1800° (πradians) with contiguous coverage as they pass over the equatorialplane.

FIG. 7—Four satellites scanning 1800° (π radians) with contiguouscoverage as they pass over the equatorial plane.

FIG. 8—A comparison of the number of satellites required forcontiguous/overlapping coverage of the earth as a function of altitudefor a zenith angle of θ_(n)=90° and constellations (n_(e)×n_(p)) asgiven here (Korb) and with the streets of coverage method.

FIG. 9—A comparison of the number of satellites required forcontiguous/overlapping coverage of the earth as a function of altitudefor a zenith angle of θ_(n)=68° and constellations (n_(e)×n_(p)) asgiven here (Korb) and with the streets of coverage method.

FIG. 10—Contiguous/overlapping areas of size θ_(x) by θ_(y) seen by aconstellation of satellites scanning a small region at mid-latitudes(for small angles θ′_(x) and θ′_(y)) where the hatched areas showoverlap in the θ_(x) direction.

FIG. 11—A factor of two overlap in the θ_(x) direction gives doublecoverage for the equatorial plane, shown in 1 dimension for a smallregion of the equatorial plane for small angles θ′_(x) and θ′_(y).

FIG. 12 a—A phase shift of one-half the coverage area θ_(x) by θ_(y) isshown between each adjacent satellite plane for the θ_(y), polar,direction.

FIG. 12 b—A phase shift of one-half the coverage area θ_(x) by θ_(y) isshown between each adjacent satellite plane for the θ_(x) longitudinal,direction.

FIG. 13—A partial constellation of 5 polar orbiting satellites is showncrossing the equatorial plane which gives contiguous/overlapping fullearth coverage in a time of one orbital period.

FIG. 14 a—A set of q satellites with an angular coverage qθ′_(x) greaterthan or equal to θ_(rot), the earth's angular rotation at the equator inthe time for one satellite orbit, is shown for a 1D linear view of theequatorial plane.

FIG. 14 b—A set of q satellites with an angular coverage qθ′_(x) greaterthan or equal to θ_(rot), the earth's angular rotation at the equator inthe time for one satellite orbit, is shown for a 2D view of theequatorial plane.

FIG. 15—A given satellite angular coverage for an elliptical orbitproduces a small (effective) angular coverage θ′_(min) at the minimumaltitude of the orbit and a large (effective) angular coverage θ′_(max)at the maximum altitude of the orbit.

FIG. 16—The latitudinal coverage −L to L for an equatorial orbit dependson the satellite's angular coverage θ_(y) and the altitude of the orbit.

FIG. 17 a—A 2D view of four satellites providing continuous, contiguouscoverage of the equatorial plane.

FIG. 17 b—A 2D view of two satellites providing coverage in a time of ½the orbital period.

FIG. 18—An inclined satellite orbit that makes an angle γ with theequatorial plane provides coverage of the earth from −γ−θ′/2 to γ+θ′/2.

FIG. 19—A 2D projection of the view on the equatorial plane is shown fora satellite with an inclined orbit that makes an angle γ with theequatorial plane and which has a cross track scan angle θ_(x) and anorbit spacing θ_(e) (for small angles θ′_(e)).

FIG. 20—The area θ″_(x) by θ″_(y) of an ultra-high performancemeasurement is contained within a portion of the coverage area θ_(x) byθ_(y) of a given satellite as it passes over the coverage area.

DETAILED DESCRIPTION Satellite Constellation Design and Configuration

There are many ways of making this scan from a satellite and using it tocover a 2 dimensional surface on the earth. In one technique, onedimension of the scan, for example the angle θ_(x) in the cross trackdirection, can be made with a scanning mechanism, while the otherdimension of the scan, for example the angle θ_(y) in the along trackdirection, can be provided by the motion of the satellite oralternatively by a scanning mechanism (FIG. 2). The along trackdirection for this example is the north-south direction, which is in thepolar plane. Thus, the satellites for this example are polar orbitingsatellites.

The cross track scan could be performed with a single detector such thatcontiguous resolution elements are scanned within a scan line. The alongtrack direction of the scan could be obtained, for example, from aseries of scan lines made with a single detector or from a series ofgroups of scan lines made with a linear detector array LDA (FIG. 3 a).The scan is made such that adjacent scan lines are contiguous. The 2dimensional scan could also be made, for example, using a lineardetector array in a push-broom configuration positioned to provide thescan information in the θ_(x) direction in a non-scanning configurationwith the satellite motion providing the scan in the θ_(y) direction(FIG. 3 b). Alternatively, a two dimensional detector array could beused with image motion compensation to provide the scan measurement withthe system in a stationary mode for a very large 2D array and wide fieldoptics or in a step scan mode with multiple positions SCi for a smallerarray (FIG. 3 c). Although the example used here is for the case ofpolar orbiting satellites, this work also applies to non-polar orbitingsatellite configurations such as equatorial orbits and orbits makingvarious angles with the equatorial plane, as will be discussed.

The basic methods for making the scan fall into two different groups.One—the cross track scan in the θ_(x) direction contains the satellite,the sub-satellite point as it intersects the earth along the nadirdirection, and the center of the earth, as shown in FIG. 1. Thesatellite motion provides the scan in the θ_(y) direction. The plane ofthe scan θ_(x) is then perpendicular to the satellite velocity vector.The projection of this scan pattern on a spherical earth is the same asthat made by the scan pattern θ′_(x), θ′_(y) centered on the earth wherethe direction of θ′_(y) is along a great circle of the earth in the sameplane as the satellite and θ′_(x) is perpendicular to θ′_(y). For thiscase, FIG. 4 shows the projection on the earth of the satellite trackand the edges of the scan at ±θ′_(x)/2 with a chord length 2X.

Two—In this case the scan plane θ_(x) is tilted relative to the nadirdirection over the range of angles from −θ_(y)/2 to +θ_(y)/2 to give arapid two dimensional scan θ_(x) by θ_(y). For θ_(y)=0, the center ofθ_(y), the projection of the scan on the earth at ±θ_(x)/2 correspondsto the chord length 2X in FIG. 1, the same points on the earth as incase 1, since these are identical projections. Also, for θ_(x)=0, thecenter of θ_(x), the projection of the scan at ±θ_(y)/2 corresponds tothe chord length 2Y, also the same two points on the earth as in case 1.For all other points, the projection of the edges of this surface on theearth is greater than that given for case 1 and the projection on theearth gives increasingly larger areas as one moves from the center ofany given edge to the corner position of that edge. Thus, it followsthat for the case of a fast scan, the projection of the scan on theearth has a field of view greater than that of case one.

A constellation of satellites is employed, each scanningcontiguous/overlapping areas of size θ_(x) by θ_(y) over the surface ofthe earth. FIG. 5 shows a 2D representation of a few of these contiguousareas for a small region near the equator for the case where the anglesθ′_(x) and θ′_(y), corresponding to θ_(x) and θ_(y), respectively, aresmall and the scanned areas are approximately rectangular in shape.

Contiguous/overlapping coverage over the earth is obtained as followsfor the general case. In the equatorial plane, the satellites are eachspaced by an approximate angle θ′_(x)

θ′_(x)=2 sin⁻¹(X/R ₀)   (1)

over at least one-half the equatorial plane where θ′_(x) is the angle asseen from the center of the earth (which corresponds to the angle θ′ inFIG. 1) and where R₀ is the radius of the earth. The number ofmeasurements needed in the equatorial plane is

n* _(e)=π/θ′_(x)   (2)

FIG. 6 shows an example for the case of 4 satellites which gives about3.7 measurements for the 180°⁰ angle used with Eq. (2). n_(e) is definedas the value of n*_(e) rounded up to the next largest integer. Thisgives the required number of polar planes or equivalently the requirednumber of satellites as they pass through the equatorial plane. Forn_(e) satellites, the angle between satellites for uniform spacing isthen given as

θ′_(x) =π/n _(e)   (³)

FIG. 7 shows an example for the case of 4 satellites. In each polarplane, the north-south direction, the measurements are approximatelyspaced by an angle θ′_(y) over the whole polar plane where θ′_(y) isdefined in an analogous manner to θ′_(x) with the chord length 2Yreplacing 2X in FIG. 1 and Eq. (1). The number of measurements needed ineach polar plane is

n* _(p)=2π/θ′_(y)   (4)

n_(p) is defined as the value of n*_(p) rounded up to the next largestinteger which gives the required number of satellites in each polarplane. For n_(p) satellites, the angle between satellites for uniformspacing is

θ′_(y)=2π/n _(p)   (5)

The number of satellites needed to obtain contiguous or overlappingcoverage over the whole earth's surface is then given as

n≧n_(e)n_(p)   (6)

Measurements are only needed over at least one-half of the equatorialplane since the measurements in the polar plane are made over the wholepolar plane and as a result fill in the back side of the equatorialplane and the back side of the earth. The angle θ′_(x) gives contiguouscoverage along the equatorial plane and overlapping coverage for allother latitudinal planes. The angle θ′_(y) gives contiguous coveragealong the north-south or polar plane direction. Alternately, themeasurements in the equatorial plane could be made over the whole plane,2π, and the measurements in the polar plane could be made over at leastone-half that plane, π.

In practice, the angle θ_(x) and θ_(y) should be chosen to provide aslightly larger coverage than θ′_(x) and θ′_(y) to give a small amountof overlapping coverage. This could be used to obtain registration ofthe images from different satellites using identification of commonground features.

The relationship between θ and θ′ is specified using a physically basedcriterion relating these angles. From FIG. 1 it can be shown that thezenith angle, the angle of a measurement at range R with the outgoingnormal to the earth's surface is

θ_(n)=θ/2+θ′/2.   (6A)

Then for any given value of θ_(n), e.g., θ_(n)<68° for downward lookingmeasurements where resolution considerations are important or θ_(n)=90°for measurements going out to the horizon, the value of θ is found fromθ with Eq. (6A). Also, one may then find the altitude z corresponding toθ and θ′ from FIG. 1 as

z=X/tan(θ/2)−z ₂   (6B)

where X is given by Eq. (1) and z₂=R₀−z₁.

One particularly important type of constellation is the symmetricalconstellation θ′=θ′_(y). From Eq. (3) the number of polar planes is

n _(e)=π/θ′  (7A)

and from Eqs. (3) and (5) the number of satellites per plane is

n_(p)=2n_(e)   (7B)

which from Eq. (6) gives

n=2n_(e) ².   (7C)

It should be noted that for θ′_(x)=θ′_(y) and for the same criterionangle θ_(n) for both the θ_(x) and θ_(y) directions, it follows thatθ_(x)=θ_(y).

The members of the symmetrical constellation may be determined from Eqs.(7A), (7B), and (7C). The first members of the constellation areprovided for convenience in Table 1. These are calculated as follows. Avalue of n_(e) is first selected, e.g., 2, θ′=90° or π/2 radians isfound from Eq. (7A), n_(p)=4 from Eq. (7B), and n=8 from Eq. (7C). Asshown in Table 1, the members of the constellation have a particularlysimple form for single fold, k=1, continuous global coverage, e.g., 2*4,3*6, 4*8, etc.

TABLE 1 Symmetrical polar constellations for k-fold continuous fullearth single k = 1, double k = 2, triple k = 3, and quadruple k = 4coverage k = 1 k = 2 k = 3 k = 4 θ′/2 n_(e) n_(P) n_(p) n_(P) n_(P) 45 24 8 12 16 30 3 6 12 18 24 22.5 4 8 16 24 32 18 5 10 20 30 40 15 6 12 2436 48 12.8571 7 14 28 42 56 11.25 8 16 32 48 64 10 9 18 36 54 72 9 10 2040 60 80 8.1818 11 22 44 66 88 7.5 12 24 48 72 96 6.923 13 26 52 78 1046.4286 14 28 56 84 112 6.0 15 30 60 90 120 5.625 16 32 64 96 128 5.294117 34 68 102 136 5.0 18 36 72 108 144 4.7368 19 38 76 114 152 4.5 20 4080 120 160 4.2857 21 42 84 126 168 4.0909 22 44 88 132 176 3.9130 23 4692 138 184 3.75 24 48 96 144 192 3.6 25 50 100 150 200 3.4615 26 523.3333 27 54 3.2143 28 56 3.0 0 60 2.9032 31 62 2.8125 32 64 2.7273 3366 2.6471 34 68 2.5714 35 70

A convenient method of finding the constellation parameters is asfollows. Given a value for θ′, X and z₁ are found from FIG. 1 and z₂ isthen found from z₁ and R₀. Then choosing a value for the altitude z, θis found from Eq. (6B) and θ_(n) from eq. (6A). Alternatively, given θ′,X, z₁ and z₂ are found as above, a value is selected for θ_(n), θ isfound from Eq. (6A) and the altitude is calculated from Eq. (6B).

FIGS. 8 and 9 show the number of satellites required for continuous,contiguous/overlapping global coverage as a function of altitude for thecases where the scan goes from nadir to a grazing angle of incidenceθ_(n)=90° with the edges of the scan for FIG. 8, a horizon scan forcommunications or missile detection, and from the nadir to θ_(n)=68° forFIG. 9, a downward looking earth based scan, see Eq. (6A). This is for arapid, continuous scan. For each constellation, the figures also givethe number of orbital planes n_(e) and the number of satellites perorbital plane n_(p) as (n_(e)×n_(p)). The constellations shown in FIGS.8 and 9 using the methods of the present invention are for thesymmetrical polar constellations shown in Table 1 and are not optimizedfor satellite phasing. These results are compared with the work of Rider(1985) who uses optimized polar orbit constellations with optimizedsatellite phasing using the “streets of coverage” technique forconstellation sizes up to approximately 160 satellites for singlecoverage. As was discussed in the background, this technique uses aconical scan pattern to scan the area within a series of minor circleson a spherical earth which are centered at each sub-satellite point.

FIG. 8 and FIG. 9 show that the constellations designed by the instantinvention, are substantially more efficient than the optimal phasedpolar constellations determined by Rider. Namely, the same coverage canbe obtained with the same altitude for constellations designed inaccordance with the method of the present invention, with about 40%fewer satellites than that of Rider. In other words, the optimal phasedRider constellations require about 1.6 times as many satellites as theconstellations of the present invention. Also, as shown in Rider (1986),his optimal polar phased constellations are significantly more efficientthan his polar unphased constellations, about a factor of 1.2, or hisoptimized inclined orbit constellations. Further, for smallconstellation sizes of about 25 satellites or less, the inclined orbitconstellations of Walker (1977) which use analytical and computer searchtechniques and constellations where each satellite can have its ownorbit plane produce about 10 to 20% more efficient designs for singlecoverage than those of Rider's optimally phased polar orbits. Then, itfollows that the constellations of the present invention provide by farthe highest efficiency of those considered for single fold, continuouscoverage.

Eqs. (1)-(6) were developed based on contiguous coverage over the wholeearth. However, if contiguous/overlapping coverage is only required forlatitudes greater than or equal to ±L₀ rather than everywhere, then thisrequirement could be met by obtaining contiguous/overlapping coverageover the latitudinal plane L₀ instead of the equatorial plane.

In this regard, it should be noted that the scan width for the set ofsatellites in a polar plane has a corresponding chord length 2X over thewhole polar plane, as best shown in FIG. 4. Then for any givenlatitudinal plane L₀, the angle of the chord length 2X as seen from thecenter of that plane is

θ′_(L) ₀ =2 sin⁻¹(X/(R ₀ cos L ₀))   (7D)

for the condition 2X≦2R₀ cos L₀ where R₀cos L₀ is the radius of thatplane. Continuous, contiguous/overlapping coverage for L≦±L₀ is thenobtained by replacing the angle θ′_(x) in Eqs. (2) and (3) by the angleθ′_(L) ₀ . For the equatorial plane, L₀=0 and Eq. (7D) reduces to Eq.(1). For all other planes, θ′_(L) ₀ ≧θ′_(x) and coverage for L≧L₀ cangenerally be achieved with a smaller number of polar planes n_(e) and asmaller number of satellites.

One method of finding the constellations for contiguous/overlappingcoverage for L≧L₀is as follows: [1] For the symmetrical constellationsθ′_(x)=θ′_(y), choose n_(e), find θ′ from Eq. (7A) and n_(p) from Eq.(7B). [2] Calculate X from Eq. (1). [3] Calculate θ′_(L) ₀ from Eq. (7D)using X from step 2. [4] Recalculate n_(e) from Eq. (2) using θ′_(L) ₀for θ′_(x). This gives the constellation parameters n_(e), n_(p), andθ′.

FIG. 10 shows a 2D representation of a few of the areas seen by theconstellation of satellites for a small region of the earth atmid-latitudes where the angles θ′_(x) and θ′_(y) are small. The areas ofcoverage by adjacent satellites overlap in the θ_(x) direction which isshown as the hatched-in (crossed) regions with a small tilt angle whichis not shown. As the satellites approach the poles, the overlap in theθ_(x) direction increases and becomes essentially complete at the polessince the orbital tracks for all satellites in the constellation crossat the poles. On the other hand, as the satellites approach the equator,the coverage for the constellation of satellites becomes contiguous asshown in FIG. 5.

For polar orbiting satellites, satellite collisions at the poles must beprevented. This is particularly important for large constellations. Thismay be addressed by using slightly different altitudes for thesatellites in different orbital planes, by using slightly differentangular deviations in the orbital track for satellites in differentorbital planes so that the satellites in different planes cross theequator at angles which differ slightly from 90°, or by offsetting thetimes at which satellites in different orbital planes pass through thepoles.

In order to allow the height of any given feature on the earth or in theatmosphere to be determined, every resolution element on the earth mustbe observed from at least two different vantage points. That is, atleast two different views must be obtained. There are two cases. One—thecase where the features are relatively stationary terrain features suchas trees, buildings, or mountains. Two or more views can be obtained forthis case from measurements of the earth built up over a period of time,e.g., days, weeks, or months, providing the satellite orbits do notfollow the same ground track. Two—for the case where the features aremoving objects (missiles, planes, trains, ships, etc.), two or moreviews must be obtained at essentially the same time, particularly forobjects moving at high velocity. This is done by overlapping the area ofcoverage for each satellite by a factor of 2 for double coverage, afactor of 3 for triple coverage, or by a factor of k for k-foldredundant coverage in either the equatorial or the polar plane. This isdone by increasing either the number of uniformly spaced orbital planesor by increasing the number of satellites per orbital plane by a factorof 2, 3, or k. This same analysis also applies to obtaining single,double, . . . k-fold coverage for all latitudes greater than any givenlatitude L₀ using Eq. (7D). FIG. 11 shows the case of double coverage inthe equatorial plane for the case where the angles θ′_(x) and θ′_(y) aresmall and where successive coverage areas are shown in one dimensiononly for clarity of presentation. Then it follows that in order toobtain 3 dimensional global stereo information with k-fold (2, 3, 4, . .. ) redundant coverage for moving objects, the total number ofsatellites needed is

n_(3d)≧kn_(e)n_(p).   (8)

Table 1, as previously discussed, gives the first 35 members of thesymmetrical polar constellations for continuous, contiguous/overlappingglobal coverage, the case of k-fold coverage with k=1. Table 1 alsogives the members of the polar constellations for k-fold (k=2, 3, or 4)redundant, global coverage for the first 25 symmetrical constellationswhere the value of n_(p) is increased to provide the desired k-foldcoverage. Alternately, the value of n_(p) could be fixed and the valueof n_(e) could be increased to give the desired coverage.

For k-fold redundant global coverage (k=2, 3, or 4) a comparison of thepolar constellations of Table 1 shows they provide considerably higherefficiency, approximately 1.4 times higher or more, than those of Rider(1985) for optimal polar phasing, those of Rider (1985) for polarunphased, or, those of Rider (1986) for optimal inclined constellations.The efficiency of the coverage for k=1 was presented and discussed forFIGS. 8 and 9. Constellations for k-fold, continuous coverage for anylatitude greater than a given latitude, L₀, can easily be calculatedusing Eq. (7D). These constellations are substantially more efficientthan the corresponding “streets of coverage” constellations given byRider (1985).

The above analysis, Eqs. (6), (7C), and (8) does not include takingadvantage of satellite phasing between adjacent satellite planes. Asdiscussed previously, for a rapid scan the smallest field of view for agiven satellite occurs at the center of each edge of the field of viewand the largest field of view occurs at the four corners of the field ofview. Then, for any given k-fold coverage, k=1, . . . m, the coverageand overlap can be increased and the number of satellites requireddecreased if in each polar plane the center of the edge of the field ofview of each satellite in that ith plane is aligned with a corner of thefield of view of the satellites in each adjacent plane i−1, and i+1 (seeFIG. 12 a). Alternatively, a similar phasing shift can be done in thelongitudinal direction by shifting the alignment of every othersatellite in the polar direction by an amount equal to one-half of thefield of view, with the shift in the longitudinal direction (see FIG. 12b). That is, in the longitudinal direction, the center of the edge ofthe field of view for each satellite is adjacent to a corner of thefield of view for another satellite. This approximately doubles thenumber of polar planes and decreases the number of satellites per planeby a factor of 2. The effects of satellite phasing are large for smallconstellations and produce large improvements in coverage.

The frame time T is the time for the sensors on a given satellite tomake a measurement over the coverage area for that satellite whichcorresponds to θ_(x) by θ_(y). If the satellite coverage areas arecontiguous or overlapping over the whole surface of the earth, the frametime corresponds to the time to provide coverage over the entire surfaceof the earth. There are two cases. One—For the case where the satelliteor instrument scan mechanism provides a rapid two dimensional scan, theframe time of the measurement is equal to the scan time and can be madeas small as desired, for example, a fraction of a second, dependent onlyon collecting sufficient signal to allow the desired measurementaccuracy and resolution to be obtained. This, however, is generally notthe case and there is often not enough signal to obtain the measurementwith the desired spatial resolution in a short frame time.

For the second case, the frame time is the time for a satellite totraverse the portion of its orbit corresponding to θ′_(y). This gives amaximum value for the frame time. From Eq. (5), the time to providecomplete earth coverage, the satellite revisit time T_(R), is

T _(R) =T _(orb) /n _(p)   (9A)

where T_(orb) is the satellite orbital period for altitude z. Therevisit time found from Eq. (9A) corresponds to the time available forthe satellite motion to provide the scan in the along track directionover the angle 2π/n_(p). For a low altitude satellite, the satelliteorbital period is of the order of 90 minutes. Then for a value of n_(p)of 20, the frame time is of the order of 4.5 minutes.

From Eq. (7C), the number of satellites required to get continuous,contiguous or overlapping coverage over the earth is quite large. Forexample, for a value of n_(e) of 5, 50 satellites are required and for avalue of n_(e) of 10, 200 satellites are required. This method alongwith the use of a rapid 2D scan allows semi-continuous measurements,e.g., of the order a fraction of a second to be obtained.

This work can also be used to develop partial satellite constellationswhich give non-contiguous coverage. In this case, for a given value ofn_(e), n_(p) can take on the value of 1, 2, . . . 2n_(e)−1 rather thanthe value of n_(p)=2n_(e) for a full (symmetric) constellation, see Eq.(7B). This gives rise to a whole set of partial constellations, one foreach value of n_(p), which provide coverage with a small number ofsatellites with revisit times as given below. For the case where thesatellite motion provides the scan in the along track direction, therevisit time is the time to cover the angular spacing betweensatellites, 2π/n_(p), and is given by Eq. (9A). This is generally amaximum value for the revisit time for a partial constellation. On theother hand, for a fast scan system over the along track satellite fieldof view θ′_(y), the minimum satellite revisit time, also called thesatellite access time, is

$\begin{matrix}{{T_{R}\left( \min \right)} = {\left( {\frac{1}{n_{p}} - \frac{\theta_{y}^{\prime}}{2\pi}} \right)\left( T_{orb} \right)}} & \left( {9B} \right)\end{matrix}$

This corresponds to the time to scan the angular separation betweensatellites minus the time to scan θ′_(y). For the case of a fullconstellation, Eq. (5), and a fast scan system, Eq. (9B) reduces toT_(R)(min)=θ as required. On the other hand, for the case where thesatellite motion provides the scan, the revisit time has a maximum valueand is given by Eq. (9A). In order to take a conservative approach, themaximum value for the revisit time, Eq. (9A), will be used for therevisit time unless otherwise stated.

As will be later shown, partial constellations have particularlyimportant applications for improving the revisit time of highperformance low altitude satellites. In addition, they can also be usedto replace high altitude geosynchronous satellites.

One example of a set of partial constellations is for a value of n_(p)of one where, from Eq. (6) there will be a total of

n≧n_(e)   (10)

satellites and the revisit time will be T_(R)=T_(orb). This is afundamentally different way of making a measurement with a much smallernumber of satellites. For a constellation with a value of n_(e) of 10,the number of satellites required for full earth coverage from Eq. (10)is only 10, whereas for a fast scan Eq. (7C) requires 200 satellites forthis case. Eq. (10) is one limiting case for n_(p)=1 and gives theminimum number of satellites needed for complete coverage of the earthwith a revisit time equal to the satellite orbital period, T_(orb). Forthis configuration, the n_(e) satellites are polar orbiting satelliteswith an angular spacing of θ′_(x) in the equatorial plane. FIG. 13 showsthe case of 5 satellites as they cross the equatorial plane providingfull earth coverage in a time T_(orb).

To obtain the desired revisit time, Eq. (9A) is used with an appropriateinteger value of n_(p). Eq. (6) then gives the number of satellitesrequired. For example, for a value of n_(p) of 10, the number ofsatellites is 10n_(e) and the revisit time is T_(orb)/10. Forapplications which require a fast semi-continuous revisit time, a valueof n_(p) approximately equal to 2n_(e) is chosen. On the other hand, forapplications which require the highest resolution and for which a frametime of the order of the orbital period is sufficient, one could choosen_(p) to be of the order of one or two and minimize the number ofsatellites which are needed.

Another orbital configuration is the case of q satellites which haveindividual coverage areas which are contiguous or overlapping with acombined angular coverage width of qθ′_(x) in the equatorial plane, asseen from the center of the earth. Then, if the coverage width, qθ′_(x),is greater than or equal to the earth's angular rotation at the equator,θ_(rot), in the time for one satellite orbit (see FIG. 14 a which showsa 1D, simplified linear view of the equatorial plane, and FIG. 14 bwhich shows a 2D view), full earth coverage is obtained in the number oforbits required to cover at least one-half of the earth's circumferenceat the equator. That is, the number of orbits is equal to π/θ_(rot)rounded up to the next largest integer. Also, the number of orbits forfull earth coverage can be reduced using multiple sets of q satellites.For example, for the case of an orbit with an altitude of approximately200 km, 124.3 mi, full earth coverage can be obtained with 1 set of 4satellites in approximately 8 orbits for an orbital period of 90minutes, or slightly longer, or in approximately 9 orbits for an orbitalperiod of slightly less than 90 minutes. This case of 4 satellitescompares with approximately 28 satellites in the configurationconsidered by Eq. (10) or with 1568 satellites in the configurationconsidered by Eq. (6) (see Table 2). For the case of low altitudesatellites where the measurements are very high performance and thesatellites are very high cost, this method provides a powerful newcapability.

These methods apply to the case of circular polar satellite orbits. Theycan be extended directly to elliptical polar orbits as well as to bothcircular and elliptical non-polar orbits. For elliptical polar orbits,contiguous or overlapping coverage over the earth is obtained by makingthe coverage between adjacent satellites in the equatorial and polarplanes contiguous, or overlapping, see Eqs. (1)-(8), where the anglesθ′now generally correspond to the location of minimum coverage in theelliptical orbit θ′_(min), which also corresponds to the location ofminimum altitude for the elliptical orbit (see FIG. 15).

These methods also apply to constellations of circular or ellipticalequatorial orbits. This allows contiguous or overlapping coverage overthat part of the earth's surface for the range of latitudes L from −L inthe southern hemisphere to +L in the northern hemisphere. The value of Ldepends on the satellite altitude and the satellite's angular coverageθ_(y). FIG. 16 shows the latitudinal planes +L and −L and the angle θ,for an equatorial orbit. There are two cases—One, contiguous oroverlapping coverage between −L and L can be obtained in the frame timeT if the spacing between adjacent satellite coverage areas in theequatorial plane is less than or equal to the size of the coverage areain that plane. This requires only a small number of satellites n. FIG.17 a shows this case for 4 satellites providing contiguous coverage overthe equatorial plane. Two, for m equally spaced satellites along theequatorial plane, where m is less than n, we can get complete coveragefrom latitude −L to L over the earth's surface in a time equal to theorbital period divided by m. This requires only a small number ofsatellites relative to the preceding case, case one. FIG. 17 b showsthis case for two satellites.

These methods also apply to constellations of circular or ellipticalorbits that make an angle γ with the equatorial plane. This allowscontiguous or overlapping coverage over that part of the earth fromapproximately −γ−θ′/2 to γ+θ′/2, see FIG. 18, where θ′/2 is theprojection of the satellite's angular coverage in the polar direction atthe maximum (or minimum) latitude of the satellite's orbit. This couldallow complete coverage of the earth for an appropriate choice of γ andθ′. There are two cases. One, contiguous or overlapping coverage can beobtained from −γ−θ′/2 to γ+θ′/2 in the frame time T using methodssimilar to those previously described for polar orbits. In this case,θ_(y) is the direction of the satellite as it makes an angle γ with theequatorial plane and θ_(x) is the scan direction perpendicular to θ_(y),see FIG. 19. The number of satellites is the number of satellites perorbital plane n_(p) as previously given, see Eq. (5), times the numberof orbital planes. The number of orbital planes is given asn_(e)=π/θ′_(e) where θ′_(e), is the angle of the orbit spacing on theequatorial plane as seen from the center of the earth and n_(e) isrounded up to an integer value. From the law of sine's for sphericaltriangles, θ′_(e) is found as θ′_(e)=sin⁻¹[sin(2X/R₀)/sin γ] for valuesof the sin θ′_(e)≦1. For small values of 2X/R₀, sin θ′≃(2X/R₀)/sin γ.This is illustrated in FIG. 19 for a 2D view for small angles Be, Two,coverage from −γ−θ′/2 to γ+θ′/2 can be obtained with m satellites perorbital plane, where m is an integer from 1 to n_(p)−1, in a time equalto the orbit time divided by m. The full and partial constellationsgiven here are very efficient for y much less than 90°.

These methods also apply to the case where the constellation of orbitsfor circular or elliptical polar orbiting satellites are rotated by anangle φ (e.g.,)90° about the polar axis such that the former north andsouth poles of the constellation make an angle 90°−φ with the equatorialplane. The two cases that occur with this configuration were discussedabove for polar orbiting satellites.

As discussed previously, k-fold continuous stereo coverage of the entireearth or coverage above any selected latitudinal plane can also beobtained for all these cases.

An ultra-high performance measurement can be made in any given area ofthe earth for a low or intermediate altitude satellite, LMAS, system asfollows. The selected area with a size that corresponds to θ″_(x) byθ″_(y), smaller than the coverage area θ_(x) by θ_(y), is acquired andscanned by a given satellite when that area first appears in a portionof the area being scanned, see FIG. 20. The coverage continues until thesatellite leaves the coverage area θ_(x) by θ_(y). As coverage of theselected area of interest ends from a given satellite (or multiplesatellites if required), coverage of the area of interest begins fromthe following satellite (or multiple satellites). This process providescontinuous coverage of the area of interest from successive satellitesuntil the coverage is stopped. The selected area can be scanned with a 2dimensional scanning system with image motion compensation in a frametime T′ which is a fraction of the full frame time. For a system with nsatellites, up to n ultra-high performance measurements of areas ofinterest could be made simultaneously, one measurement for eachsatellite. Also, if required, one or more ultra high performancemeasurements could be made by a given satellite within the area θ_(x) byθ_(y) using a step scan system. The ultra-high performance system isable to take advantage of all the features of a given LMAS system, andhas the advantage that it only has to scan a small fraction of the areaof a given system.

The only loss of coverage from using the ultra-high performance featureis in the single area θ_(x) by θ_(y) containing the feature of interest.For an LMAS system with 200 satellites, this would result in a loss ofcoverage of the order of 1/200 or 0.5% of the coverage of the earth. Incontrast, the use of a zoom type feature for a 3 satellite GS systemwould result in a loss of ⅓ of the data over the earth which severelylimits the use of this feature. Also, an LMAS system has a loweraltitude than a GS system. This allows a much higher diffraction limitedresolution than a GS system, e.g., more than 100 times higher resolutionfor a 200 km altitude LMAS system and the same diameter telescopes.These features will be discussed later and presented with numericalexamples.

The ultra-high performance system allows an essentially continuousobservation of a given area with frame time T′ which is a real timecapability that can be called on demand. For example, the ultra-highperformance scan pattern over any particular small region, for example,a 50 mile by 75 mile region in China or the polar region, could be setand begin to be utilized using pre-programmed methods in a matter ofseconds. This would provide ultra-high performance, essentiallycontinuous, real time images of the particular area of interest, muchlike a movie for a single spectral band or for any combination ofspectral bands. This capability would provide information that could noteasily be acquired any other way. For example, even if one launched aspecial satellite to attempt to provide this information, measurementsof the particular area of interest might be acquired only once per weekor once per several days versus essentially continuous observations.

Variations of constellation designs herein disclosed can be usedseparately or together in a large number of ways to accomplish differentobjectives. For example, polar satellite constellations can be used toprovide full earth k-fold contiguous/overlapping coverage in the frametime T, e.g., in times of a fraction of a second. However, if k-foldcontiguous/overlapping coverage is only required for latitudes greaterthan ±L₀, then this requirement could be met using the method of Eq.(7D). Alternately, one could use a small constellation of equatorialsatellites to supplement the data from this case. This could providek-fold continuous, contiguous/overlapping data for latitudes 0 to L₀from the equatorial satellite constellation which could be used with thek-fold continuous polar constellation data for L≧L₀ to provide k-foldcontinuous coverage over the earth.

On the other hand, (if k-fold coverage in the polar region and at highlatitudes is not a priority requirement), one can use a constellation ofsatellites with orbits inclined at an angle γ to the equatorial plane.As previously described, this gives contiguous/overlapping coverage inthe frame time T from −γ−θ′/2 to γ+θ′/2 and also gives high densityoverlapping coverage in the region of the latitudinal planes γ±θ′/2 and−γ±θ′/2.

If k-fold contiguous/overlapping coverage of the earth in the frame timeT is only required in a given latitudinal band in the Northern andSouthern Hemisphere, this can be achieved as follows. The design of theconstellation of satellites with orbits inclined at an angle γ to theequatorial plane is chosen to give k-fold contiguous/overlappingcoverage over the latitudinal plane L₀, instead of the equatorial plane.This gives k-fold contiguous/overlapping coverage for the latitudinalband L₀≦L≦γ+θ′/2 in the Northern Hemisphere and the correspondinglatitudinal band in the Southern Hemisphere.

The methods disclosed herein may be employed to image the whole earth orportions thereof from satellites with high spatial resolution with oneor more sensors in a short frame time which can produce high data rates.On-board data compression or analysis techniques may be required toreduce the data rate because of bandwidth limitations in transmittingdata from satellites to earth-based data receiving stations. The datarate may also be reduced by transmitting data only for particular areasof the earth, for selected time periods, or for data with selectedproperties, such as particular altitudes, velocities, or spectralproperties. For example, if the user's interest was in detecting areaswith a particular velocity or altitude range, then the data could beprocessed on board to determine those areas, and only the data which metthe particular altitude or velocity criteria would be transmitted toearth based data receiving stations.

For a full constellation of satellites, an LMAS system has thecapability of providing a very high data rate satellite to groundcommunication system. This occurs since a full LMAS constellationprovides contiguous or overlapping coverage of the entire earth. As aresult, it has a direct line of sight with every point on the earth atall times and thus can see every earth based data receiving station. Ifin addition, the angles θ_(n), see Eq. (6A), for each satellite arechosen such that θ_(n)≦90° at the mid-point between adjacent satellites,then there is also a direct line of sight between adjacent satellites.Every satellite can then see at least 3 other satellites. Data can thenbe transferred between satellites, as needed, and from there to theground using the entire set of ground based receiving stations.

A full constellation LMAS system also provides a powerful generalizedhigh data rate communications system. This system could directly receivedata from and transmit data to a small, low power, mobile (orstationary) cell phone transmitting and receiving station for a givenarea or even directly to individual cell phone units. Theses areimportant new capabilities. This system would use the direct line ofsight of the LMAS system with every point of the earth for receivingcellular calls from the earth. It would use the direct line of sightbetween adjacent satellites to transfer calls from a given receivingsatellite to the appropriate transmitting satellite with a line of sightview of the receiving cell phone area. It would then transfer the callto the receiving cell phone. An LMAS system has a very low altitudecompared to a conventional geosynchronous or low altitude communicationssystem. As a result, it has a signal advantage compared to aconventional system equal to the ratio of the squares of the slant pathsfrom the ground transmitter to the receiving satellite for the twopaths. For an LMAS system with an altitude of 250 mi or 124 mi, thiscorresponds to a signal advantage versus a GS system of approximately8,000 or 32,000 times for the nadir direction. For an LMAS system at 124mi vs. a system at 500 mi altitude, this corresponds to a signaladvantage of approximately 16 times.

Performance Evaluation

Instrument performance models are developed for both active and passivesatellite systems for the general case where the noise is containedwithin the signal and also for the case where the noise is independentof the signal. Cost models are developed which relate the instrumentperformance parameters, the constellation size, and the system cost. Thecost models are combined with the performance and the constellationmodels and are applied to the problem of improving the performance ofgeosynchronous satellite systems and also high performance low altitudesatellite systems. The cases of both signal limited and diffractionlimited performance are treated.

The performance and cost models developed here generally use ratiotechniques. This allows the performance and cost of systems using themethods given here to be directly compared with the performance and costof current systems. Thus, the user of these methods can use his past andcurrent performance and cost information to directly determine the costand performance of systems using the methods given here.

The ratio technique given here for comparing the performance ofsatellite instruments at different altitudes also minimizes the effectsof the atmosphere on the comparison. To further reduce the effects ofthe atmosphere, measurements are compared at two altitudes for the sameangular path through the atmosphere. In this case, the atmosphericeffects on the measurement, that is, the effects of atmospherictransmission, scattering losses, and path radiance, cancel to firstorder. To obtain the same atmospheric path from the scene to thesatellite, the angle of that path relative to the normal to the earth'ssurface, θ_(n)=(θ+θ′)/2, must be the same for the two measurements,although the individual angles θ and θ′ may be quite different for thetwo measurements. Also, in the solar reflected region, the angle of thesun relative to the normal to the earth's surface on the incoming pathfrom the sun to the scene should be the same for the two measurements.These conditions are met approximately for the comparisons set forthherein at the nadir and at the edge of the scan.

1. Signal (Signal to Noise) Limited Performance—Geosynchronous and HighAltitude Satellites

For a passive instrument observing a scene of radiance L, either solarreflected/scattered radiation or thermally emitted radiation, the signalS collected by the receiver in terms of energy is

S ∝ A_(c)β² Δt   (11)

where A_(c) is the collector area, β is the angular resolution of theinstrument, and Δt is the measurement time per resolution element. Now βis given as

β=d cos θ_(n) /R   (12)

where d is the size of the resolution element projected on the earth, Ris the distance, the slant range, from the receiver to the spatialelement being measured and θ_(n) is the zenith angle between theoutgoing normal to the earth and R. It is noted that β=d/R for θ_(n)=0.Then, the ratio of the angular resolution for altitude z, β_(z), andthat for GS orbit, β_(g), where the subscript g designates GS orbit is

$\begin{matrix}{\frac{\beta_{z}}{\beta_{g}} = {\frac{d_{z}}{d_{g}}\frac{R_{g}}{R_{z}}\sqrt{A}}} & (13) \\{where} & \; \\{A = {\cos^{2}{\theta_{n_{z}}/\cos^{2}}{\theta_{n_{g}}.}}} & \left( {13A} \right)\end{matrix}$

The subscript g could also generally be used to designate any otheraltitude z.

For a measurement area corresponding to the angular field of view θ_(x)in one direction and θ_(y) in the other direction, the number ofresolution elements corresponding to the angular field θ_(x) is

n _(x)≃θ_(x)/β  (14)

which is the angular field divided by the angular resolution for oneresolution element. Similarly, for θ_(y) the number of resolutionelements is

n_(y)≃θ_(y)/β  (15)

which is the angular field divided by the angular resolution per elementor alternatively the angular field divided by the angular resolution perline which gives the number of lines in θ_(y). Then the number ofresolution elements n_(s) in the coverage area θ_(x) by θ_(y) is

n_(s)≃θ_(x)θ_(y)/β²   (16)

If this measurement is made in the frame time T, the time to measure theregion θ_(x) by θ_(y), then the measurement time per resolution elementfor a single detector is

$\begin{matrix}\begin{matrix}{{\Delta \; t} = {T/n_{s}}} \\{\simeq \frac{T\; \beta^{2}}{\theta_{x}\theta_{y}}}\end{matrix} & (17)\end{matrix}$

The same detector or detector array, in terms of the number of elements,is used for measurements from different orbits, that is for altitude zand for GS orbit. The detector for each system then provides the sameadvantages so that the detector or detector array does not give eithersystem any inherent advantage. For a detector array which has a size ofp by q′ elements and which is used optimally then the total timeavailable for each measurement will be increased by a factor pq′ and

$\begin{matrix}{{\Delta \; t} \simeq {{pq}^{\prime}\frac{T\; \beta^{2}}{\theta_{x}\theta_{y}}}} & (18)\end{matrix}$

If the field of view of the detector array is ≦θ_(x) by θ_(y) foraltitudes z and g, then for measurements with the same array size andframe time

$\begin{matrix}{{\frac{\Delta \; t_{z}}{\Delta \; t_{g}} = {\left( \frac{\beta_{z}}{\beta_{g}} \right)^{2}R^{*}}}{with}} & (19) \\{R^{*} = \frac{\theta_{x,g}\theta_{y,g}}{\theta_{x,z}\theta_{y,z}}} & (20)\end{matrix}$

There are two different cases to be considered. One, the case of shotnoise which is described by Poisson statistics where the noise source iscontained within the signal. Two, the case where the noise isindependent of the signal. For shot noise, the noise N is given as

N ∝ √{square root over (S)}  (21)

and the signal to noise ratio S/N is

S/N ∝ √{square root over (S)}  (22)

For the case of shot noise, the ratio of the signals at altitude z andGS altitude g is found from Eqs. (11), (13), and (19), for telescopes ofdiameter D_(z) and D_(g), as

$\begin{matrix}{\frac{S_{z}}{S_{g}} = {\left( \frac{D_{z}}{D_{g}} \right)^{2}\left( \frac{_{z}}{_{g}} \right)^{4}\left( \frac{R_{g}}{R_{z}} \right)^{4}R^{*}A^{2}}} & (23)\end{matrix}$

for small angles, θ is found from FIG. (1) as

θ≃2X/R   (24)

and from Eq. (20) for θ_(x)=θ_(y) and for small angles, R* is

$\begin{matrix}{R^{*} \simeq {\left( \frac{X_{g}}{X_{z}} \right)^{2}\left( \frac{R_{z}}{R_{g}} \right)^{2}}} & (25)\end{matrix}$

Then for small angles θ the signal ratio is found from Eqs. (23) and(25) as

$\begin{matrix}{\frac{S_{z}}{S_{g}} \simeq {\left( \frac{D_{z}}{D_{g}} \right)^{2}\left( \frac{_{z}}{_{g}} \right)^{4}\left( \frac{R_{g}}{R_{z}} \right)^{2}\left( \frac{X_{g}}{X_{z}} \right)^{2}A^{2}}} & (26)\end{matrix}$

which agrees with a direct analysis for the case of small angles. Eq.(23) gives the signal ratio for measurements at altitude z compared withthose at any other altitude g, for example, those at GS altitude. Formeasurements with the same resolution

d_(z)=d_(g)   (27)

Eq. (23) gives

$\begin{matrix}{\frac{S_{z}}{S_{g}} = {\left( \frac{D_{z}}{D_{g}} \right)^{2}\left( \frac{R_{g}}{R_{z}} \right)^{4}R^{*}A^{2}}} & (28)\end{matrix}$

To evaluate the signal to noise for the case of shot noise, it followsfrom Eqs. (22) and (23) that

$\begin{matrix}{\frac{\left( {S/N} \right)_{z}}{\left( {S/N} \right)_{g}} = {\left( \frac{D_{z}}{D_{g}} \right)\left( \frac{_{z}}{_{g}} \right)^{2}\left( \frac{R_{g}}{R_{z}} \right)^{2}\left( R^{*} \right)^{\frac{1}{2}}A}} & (29)\end{matrix}$

and for the same resolution

$\begin{matrix}{\frac{\left( {S/N} \right)_{z}}{\left( {S/N} \right)_{g}} = {\frac{D_{z}}{D_{g}}\left( \frac{R_{g}}{R_{z}} \right)^{2}\left( R^{*} \right)^{\frac{1}{2}}A}} & (30)\end{matrix}$

Then for the case of the same signal to noise ratio, it follows from Eq.(29) that

$\begin{matrix}{\left( \frac{_{g}}{_{z}} \right)^{2} = {\frac{D_{z}}{D_{g}}\left( \frac{R_{g}}{R_{z}} \right)^{2}\left( R^{*} \right)^{\frac{1}{2}}A}} & (31)\end{matrix}$

Eq. (31) gives the ratio of the square of the resolutions for GSaltitude and altitude z, respectively. This is also the ratio of thearea of a spatial resolution element measured from GS altitude to thatat altitude z. Thus, Eq. (31) is a direct measure of the informationcontent of the two measurements. Eq. (31) holds assuming the availablesignal limits the resolution and not the diffraction limit. From Eq.(25) for small angles and θ_(x)=θ_(y), Eq. (29) reduces to

$\begin{matrix}{\frac{\left( {S/N} \right)_{z}}{\left( {S/N} \right)_{g}} \simeq {\frac{D_{z}}{D_{g}}\left( \frac{_{z}}{_{g}} \right)^{2}\frac{R_{g}}{R_{z}}\frac{X_{g}}{X_{z}}A}} & (32)\end{matrix}$

and similarly Eq. (31) for the ratio of the square of the resolutionsreduces to

$\begin{matrix}{\left( \frac{_{g}}{_{z}} \right)^{2} \simeq {\frac{D_{z}}{D_{g}}\frac{R_{g}}{R_{z}}\frac{X_{g}}{X_{z}}A}} & (33)\end{matrix}$

Eqs. (32) and (33) agree with an analysis for the case of small angles.

The altitude for a GS orbit is much greater than for a low or midaltitude orbit and as a result R_(g)/R_(z) will be much larger than one.Also, X_(g)/X_(z) will also be much larger than one. It then followsfrom Eq. (23) and Eq. (26) that the signal ratio for altitude z and forGS altitude will be much greater than one for the same resolution andtelescope diameter and there will be a large performance advantage formeasurements from a low or mid altitude satellite LMAS system, ascompared to a system at GS altitude. Similarly, the same termsR_(g)/R_(z) and X_(g)/X_(z) occur in Eqs. (30) and (32) for the ratio ofthe signal to noise terms and in Eqs. (31) and (33) for the ratio of thesquare of the resolutions. Then, there will also be a large advantagefor using a low or mid altitude system for the signal to noise, as wellas for the spatial resolution and information content measurements,compared to a GS orbital system. As hereinafter set forth, thesequalitative findings are strongly supported by numerical calculations.Further, as will be shown high levels of performance may be obtained atthe same or lower cost as a low performance GS system. Even a smallimprovement in information content, for example a 30 or 40% improvement,is generally considered to be of significant importance.

For the case where the noise in the measurement is independent ofsignal, Eq. (11) for the signal may be expressed in terms of the poweras

$\begin{matrix}{P^{\prime} = {\frac{S}{\Delta \; t} \propto {A_{c}\beta^{2}}}} & (34)\end{matrix}$

The noise in terms of the power is given as

$\begin{matrix}{N \propto \left( {A_{d}\Delta \; f} \right)^{\frac{1}{2}}} & (35)\end{matrix}$

where Δf is the bandwidth of the measurement which is given as

Δf ∝ 1/Δt   (36)

where Δt is the measurement time. The detector area A_(d) is given as

A_(d) ∝(FβD)²   (37)

where F is the f number of the system which is the ratio of the focallength of the collector to its diameter. Then from Eqs. (35), (36), and(37), the noise is given as

$\begin{matrix}{N \propto {F\; \beta \; {D/\left( {\Delta \; t} \right)^{\frac{1}{2}}}}} & (38)\end{matrix}$

and the signal to noise ratio, the signal power P′, Eq. (34) to thenoise power N, Eq. (35), is

$\begin{matrix}{{S/N} \propto {D\; {{\beta \left( {\Delta \; t} \right)}^{\frac{1}{2}}/F}}} & (39)\end{matrix}$

Then the ratio of the signal to noise at altitude z and GS altitude isfound from Eqs. (39), (13), (19), and (20) as

$\begin{matrix}{\frac{\left( {S/N} \right)_{z}}{\left( {S/N} \right)_{g}} = {\frac{F_{g}}{F_{z}}\frac{D_{z}}{D_{g}}\left( \frac{d_{z}}{d_{g}} \right)^{2}\left( \frac{R_{g}}{R_{z}} \right)^{2}\left( R^{*} \right)^{\frac{1}{2}}A}} & (40)\end{matrix}$

and for the same values of F at altitudes z and g, this reduces to

$\begin{matrix}{\frac{\left( {S/N} \right)_{z}}{\left( {S/N} \right)_{g}} = {\frac{D_{z}}{D_{g}}\left( \frac{d_{z}}{d_{g}} \right)^{2}\left( \frac{R_{g}}{R_{z}} \right)^{2}\left( R^{*} \right)^{\frac{1}{2}}A}} & (41)\end{matrix}$

which is the same result as for shot noise, see Eq. (29). For systemswith the same resolution Eq. (41) reduces to

$\begin{matrix}{\frac{\left( {S/N} \right)_{z}}{\left( {S/N} \right)_{g}} = {\frac{D_{z}}{D_{g}}\left( \frac{R_{g}}{R_{z}} \right)^{2}\left( R^{*} \right)^{\frac{1}{2}}A}} & (42)\end{matrix}$

which is also the same result as for shot noise, see Eq. (30). Forsystems with the same signal to noise ratio, Eq. (41) gives

$\begin{matrix}{\left( \frac{d_{g}}{d_{z}} \right)^{2} = {\frac{D_{z}}{D_{g}}\left( \frac{R_{g}}{R_{z}} \right)^{2}\left( R^{*} \right)^{\frac{1}{2}}A}} & (43)\end{matrix}$

which is the same result as for shot noise, see Eq. (31).

Thus, for the case where the noise is independent of signal, the ratioof the performance at altitude z and GS altitude is the same as for shotnoise providing the systems at the two altitudes use the same F number.Then the results for noise independent of signal are given by theresults for shot noise.

For measurements with an active system, for example a laser systememitting a pulse of energy E, the signal collected at the receiver for aresolution d at range R after scattering off the atmosphere from a rangegate of thickness ΔR (much greater than the transmitter equivalent pulselength) is

$\begin{matrix}{S \propto {\frac{{EA}_{c}}{R^{2}}\Delta \; R}} & (44)\end{matrix}$

where ΔR=Δz/cos θ_(n) and Δz is the vertical resolution. Eq. (44) is fora receiver angular field of view aligned with and greater than that ofthe transmitter. Then, for measurements with the same verticalresolution Δz_(g)=Δz_(z), it follows from Eq. (44) that the signal ratiofor altitude z and GS altitude is

$\begin{matrix}{{\frac{S_{z}}{S_{g}} = {\frac{E_{z}}{E_{g}}\left( \frac{D_{z}}{D_{g}} \right)^{2}\left( \frac{R_{g}}{R_{z}} \right)^{2}A^{*}}}{where}} & (45) \\{A^{*} = {\cos \; {\theta_{n_{g}}/\cos}\; \theta_{n_{z}}}} & (46)\end{matrix}$

and for the case of equal signal, S_(z)=S_(g), it follows from Eq. (45)that

$\begin{matrix}{\frac{E_{g}}{E_{z}} = {\left( \frac{D_{z}}{D_{g}} \right)^{2}\left( \frac{R_{g}}{R_{z}} \right)^{2}A^{*}}} & (47)\end{matrix}$

Then for the same signal level and diameter telescopes, the measurementfrom a low or mid altitude system has an advantage over a GS orbitmeasurement system, in terms of the energy of the active system, by thefactor A*(R_(g)/R_(z))² which is a very large factor.

Alternatively, if the active systems have the same total energy outputper frame time T from altitude z and GS altitude, then

E _(z) n _(s)(z)=E _(g) n _(s)(g)   (48)

where n_(s) is the number of resolution elements in the angular fieldθ_(x) by θ_(y), see Eq. (16), measured in the frame time T. Then theratio of the energies at altitude z and GS altitude is found from Eqs.(48), (13), (16), and (20) as

$\begin{matrix}{\frac{E_{z}}{E_{g}} = {\left( \frac{d_{z}}{d_{g}} \right)^{2}\left( \frac{R_{g}}{R_{z}} \right)^{2}R^{*}A}} & (49)\end{matrix}$

and the signal ratio is found from Eqs. (45) and (49) as

$\begin{matrix}{\frac{S_{z}}{S_{g}} = {\left( \frac{D_{z}}{D_{g}} \right)^{2}\left( \frac{d_{z}}{d_{g}} \right)^{2}\left( \frac{R_{g}}{R_{z}} \right)^{4}R^{*}{AA}^{*}}} & (50)\end{matrix}$

For the case where the resolution is the same for altitude z and GSaltitude, then the signal ratio for an active system, Eq. (50), and thesignal ratio for a passive system for shot noise, Eq. (23), are the samewithin a factor of A*/A. Also, the ratio of the signal to noise ratiosfor active and passive systems, Eq. (29), are also the same within afactor of √{square root over (A*/A)} for this case. However, for thecase where the signal to noise ratio at altitudes z and g are the same,then it follows from Eqs. (50), (22), and (31) that

$\begin{matrix}{\left( \frac{d_{g}}{d_{z}} \right)_{act} = {\sqrt{\frac{A^{*}}{A}}\left( \frac{_{g}}{_{z}} \right)_{pas}^{2}}} & (51)\end{matrix}$

where act and pas designate active and passive systems, respectively.Thus the resolution improvement for an active system at any twoaltitudes g and z varies close to the square of the resolutionimprovement for a passive system. As a consequence, the resolutionimprovement for an active system is much larger than for a passivesystem.

2. Cost Analysis

Cost models can be constructed and used with performance models asfollows. The cost c of a satellite system with n satellites withtelescope diameter D can be represented by

c ∝ f(D)g(n)   (52)

where the functions f and g represent the functional relationship of thecost on the telescope diameter and on the number of satellites. For asystem of n_(z) satellites with an altitude (configuration) z withtelescope diameter D_(z) and a second system of n_(g) satellites with analtitude (configuration) g with telescope diameter D_(g), the costs arerelated by

$\begin{matrix}{\frac{c_{g}}{c_{z}} = \frac{{f\left( D_{g} \right)}{g\left( n_{g} \right)}}{{f\left( D_{z} \right)}{g\left( n_{z} \right)}}} & (53)\end{matrix}$

Eq. (53) can be used to represent the cost even for the most complexsystems. For systems where the cost varies as D^(a) and n^(b), itfollows from Eq. (53) that

$\begin{matrix}{\frac{D_{g}}{D_{z}} = {\left( \frac{c_{g}}{c_{z}} \right)^{\frac{1}{a}}\left( \frac{n_{z}}{n_{g}} \right)^{\frac{b}{a}}}} & (54)\end{matrix}$

For high technology satellites, the parameter a is often found to beapproximately 3 and the cost varies as D³. There are generally alsoeconomies of scale in building a large number of identical units and asa result the parameter b may be much less than one. However, for a worstcase scenario where the cost is proportional to the number ofsatellites, Eq. (54) reduces to

$\begin{matrix}{\frac{D_{g}}{D_{z}} = {\left( \frac{c_{g}}{c_{z}} \right)^{\frac{1}{3}}\left( \frac{n_{z}}{n_{g}} \right)^{\frac{1}{3}}}} & (55)\end{matrix}$

The approach to selecting the orbital characteristics for a givenmeasurement, the satellite altitude and the number of satellites to beused, has generally been determined by historical precedence. Thisdetermines the system design as follows. One—to minimize the cost of asatellite system, the number of satellites used is minimized since thecost of a system increases directly with the number of satellites.Two—One of the main methods for improving the performance of a system isto increase the telescope diameter. This methodology produces thefollowing result. A relatively small increase in telescope diameter (afactor of 2) produces a large increase in cost (a factor of 8 for c=D³,see Eq. (55)) and gives a relatively small improvement in performance (afactor of 2^(1/2)=1.4 for a passive system, see Eqs. (31) or (43)).

Various performance/constellation/cost methods are possible based on themethods of the performance evaluation, see for example Eqs. (31), (43),and (50), the cost/performance models Eqs. (53) to (55), and the numberof satellites needed for any given constellation design, see for exampleEqs. (1)-(8). One methodology used here utilizes tradeoffs between theperformance of a satellite system (e.g., the resolution or the signal tonoise ratio), the altitude of the satellite system, the angle θ_(n), thenumber of satellites required to meet a given measurement objective(e.g., one—near simultaneous, contiguous measurements over the wholeearth; two—high resolution measurements with fast revisit times for anypoint on the earth; etc . . . ), the ratio of the telescope diameters atthe two altitudes being considered, and the resulting cost. Inparticular, it is generally found here that much higher performance canbe obtained using low altitude satellites with small angles θ_(n) forany given measurement as opposed to high altitude satellites with largeangles θ_(n) as is generally used. This, however, requires a largeincrease in the number of satellites required to make that measurementand from Eq. (55), a corresponding increase in cost. This increase incost can be offset, however, by using smaller diameter measurementsystems. The resulting system has a relatively low altitude with small(zenith) angles θ_(n) and uses a large constellation of satellites withsmall telescope diameter measurement instruments. The net performance ofthis system is much higher than that of a (relatively) high altitudesystem with large telescopes and one or a few satellites. Also, the costof this low altitude system is the same or much less than that of a highaltitude system. As will be shown in the performance analysis anddiscussion section, these methods allow a very large number of differentsystems to be designed with much higher performance and with the same orlower cost as current systems.

3. Signal (Signal to Noise) Limited Low Altitude Satellites

The methods of this section can also be used to improve the revisit timeof high performance, low altitude satellites with non-contiguous,periodic coverage of the earth, approximately once every 12 hours. Theperformance analysis for non-contiguous coverage using partial satelliteconstellations, see paragraphs 90 to 93, is essentially identical to theprior analysis of this section (for contiguous/overlapping nearsimultaneous coverage of the earth using full satellite constellations)except for the effect of the frame time on the measurement. In the caseof non-contiguous coverage, the frame time varies with altitude whereasin the case of contiguous coverage it does not. The analysis whichfollows accounts for the effects of the frame time varying withaltitude.

The performance of a measurement system depends on the measurement timeΔt, see Eq. (11), which is proportional to the frame time T, see Eq.(18). The ratio of the measurement times for altitudes z and g is givenby Eq. (19) for the case where the frame times at altitudes z and g areassumed to be equal and thus cancel. For the case where the frame timesat altitudes z and g are T_(z) and T_(g), it follows from Eq. (18) thatthe ratio of the measurement times at altitudes z and g is equal toT_(z)/T_(g) times the right side of Eq. (19) which is

$\begin{matrix}{\frac{\Delta \; t_{z}}{\Delta \; t_{g}} = {\left( \frac{\beta_{z}}{\beta_{g}} \right)^{2}R^{*}\frac{T_{z}}{T_{g}}}} & \left( {19A} \right)\end{matrix}$

It then follows from Eqs. (11) and (19A) that the signal ratio at thetwo altitudes z and g is equal to T_(z)/T_(g) times the right side ofEq. (23). Also, it follows from Eq. (22) and Eq. (31) that the ratio ofthe squares of the resolutions at altitudes g and z for the same signalto noise ratio, is equal to (T_(z)/T_(g))^(1/2) times the right side Eq.(31) which is

$\begin{matrix}{\left( \frac{d_{g}}{d_{z}} \right)^{2} = {\frac{D_{z}}{D_{g}}\left( \frac{R_{g}}{R_{z}} \right)^{2}\left( R^{*} \right)^{\frac{1}{2}}\left( \frac{T_{z}}{T_{g}} \right)^{\frac{1}{2}}A}} & \left( {31A} \right)\end{matrix}$

The frame time, as discussed in paragraph 88, is the time for asatellite to traverse its along-track angular field θ′_(y), as seen fromthe center of the earth, which is

$\begin{matrix}{T = {\frac{\theta_{y}^{\prime}}{2\pi}T_{orb}}} & (56)\end{matrix}$

where T_(orb) is the satellite's orbital period. For a satellite in acircular orbit, T_(orb) is found by setting the gravitational forceequal to the centripetal, centrifugal, force which gives

$\begin{matrix}{T_{orb} = {\frac{2\pi}{R_{o}\sqrt{g_{o}}}\left( {R_{o} + z} \right)^{\frac{3}{2}}}} & (57)\end{matrix}$

where R₀+z is the distance between the satellite and the center of theearth, and g_(o) is the acceleration of gravity at the surface of theearth. Then it follows from Eqs. (31A), (56), and (57) withθ′_(z)=θ′_(y,z)=θ′_(x,z) that

$\begin{matrix}{\left( \frac{d_{g}}{d_{z}} \right)^{2} = {\frac{D_{z}}{D_{g}}\left( \frac{R_{g}}{R_{z}} \right)^{2}\left( R^{*} \right)^{\frac{1}{2}}\left( \frac{\theta_{z}^{\prime}}{\theta_{g}^{\prime}} \right)^{\frac{1}{2}}\left( \frac{R_{o} + z}{R_{o} + g} \right)^{\frac{3}{4}}A}} & (58)\end{matrix}$

and from Eq. (58) substituting for R* with θ_(x)=θ_(y)=θ for symmetricalconstellations gives

$\begin{matrix}{\frac{D_{g}}{D_{z}} = {\left( \frac{d_{z}}{d_{g}} \right)^{2}\left( \frac{R_{g}}{R_{z}} \right)^{2}\frac{\theta_{g}}{\theta_{z}}\left( \frac{\theta_{z}^{\prime}}{\theta_{g}^{\prime}} \right)^{\frac{1}{2}}\left( \frac{R_{o} + z}{R_{o} + g} \right)^{\frac{3}{4}}A}} & (59)\end{matrix}$

D_(g)/D_(z) is related to the number of satellites by Eq. (55) which,for the same cost, gives the number of satellites at altitude z relativeto those at altitude g, as

$\begin{matrix}{n_{z} = {n_{g}\left( \frac{D_{g}}{D_{z}} \right)}^{3}} & (60)\end{matrix}$

For a large value of D_(g)/D_(z), it follows that n_(z) is large for thesame cost as n_(g), e.g., one satellite. The revisit time is then givenby Eq. (9A) as

$\begin{matrix}{T_{R} = \frac{T_{orb}}{\left( \frac{n}{n_{e}} \right)}} & (61)\end{matrix}$

for n_(z)≧n_(e) where n_(e) is the number of orbital planes, see Eq.(3), and where n/n_(e) is the number of satellites per orbital plane.This is for integer values for n_(z) and n/n_(e). This can be achievedfor n_(z) by rounding the value of n_(z) down to the next lowestinteger. Integer values of n/n_(e) can be achieved by adjusting theparameters to obtain values of n/n_(e) slightly greater than or equal toan integer value and then rounding down to an integer value as above.

The revisit time is greatly reduced for large values of D_(g)/D_(z). Eq.(59) shows that D_(g)/D_(z) depends on the square of R_(g)/R_(z),d_(z)/d_(g) and on A. It then follows that the revisit time can bereduced by:

-   -   1) Reducing the altitude z relative to the reference altitude g        of about 500 mi which is generally currently used for low        altitude satellites.    -   2) Decreasing θ_(n), relative to the reference value of θ_(n)        _(g) of about 68° at 500 mi altitude.    -   3) Decreasing the resolution of measurements at altitude z        relative to the resolution at altitude g.

Alternately, the improved performance which results from steps 1) and 2)above can be used to improve the spatial resolution or the signal tonoise ratio of the measurements. The Performance Analysis and Discussionsection will show how these improvements are made and gives typicalresults.

4. Diffraction Limited Performance

The prior work in sections 1 and 3 is for the case where the satelliteperformance is signal limited. That is, the signal to noise ratio, theresolution, the measurement time and the revisit time are limited by theavailable signal. A second case is for diffraction limited performance.In this case, the resolution is limited by the size of the telescopeaperture.

The theoretical limit for the resolution is given by the diffractionlimit, which for a circular aperture is

$\begin{matrix}{\beta_{d} = {1.22\frac{\lambda}{D}}} & (62)\end{matrix}$

which yields a resolution of

d _(d)=1.22λR/(D cos θ_(n))   (63)

Then the ratio of the diffraction limited resolution for altitude g andany altitude z is

$\begin{matrix}{\frac{d_{d,g}}{d_{d,z}} = {\frac{D_{z}}{D_{g}}\frac{R_{g}}{R_{z}}\sqrt{A}}} & (64)\end{matrix}$

The diffraction limited resolution is generally the highest resolutionwhich can be achieved, independent of the available signal, with theexception of a synthetic aperture radar or phased array type system.

The Performance Analysis and Discussion section will give results forthis case.

Performance Analysis and Discussion

The performance, constellation, and cost analysis methods developed inthis work are applied to the problem of improving the performance ofmajor satellite systems. These methods are used to show how theperformance of sensors on geosynchronous and high altitude satellites,and the revisit time for full earth coverage for low altitude satellitescan be greatly improved. Methods are also presented for achievingcontinuous coverage or much higher resolution for high performance, lowaltitude satellites. Both signal limited and diffraction limitedperformance are presented and compared for each of the applicationsabove.

Table 2 shows some examples of satellite orbital and instrumentparameters for the case of symmetrical constellations withθ′_(x)=θ′_(y). These constellations have contiguous/overlapping fullearth coverage and n_(e) and n_(p) values of 2×4 at 6000 mi altitude,3×6 at 3000 mi altitude, 6×12 at 1000 mi altitude, 9×18 at 500 mialtitude, 13×26 at 250 mi, and 28×56 at 124.27 mi. The parameters showninclude the satellite altitude, the scan angle θ, the scan angle θ′asseen form the center of the earth, the number of satellites n needed toobtain complete coverage of the earth in the frame time T, a time of theorder of a fraction of a second to thousands of seconds, and the chordlength of the measurement, 2X. The angle θ′ is also the satelliteangular spacing as seen from the center of the earth, see Eqs. (3) and(5).

TABLE 2 Scan Angle θ, Scan Angle θ′ as Measured from the Center of theEarth, Number of Satellites (n), and Chord Length of Measurements (2X)for Various Satellite Altitudes (z). z (mi) θ/2 (deg) θ′/2 (deg) n$\frac{\theta + \theta^{\prime}}{\begin{matrix}2 \\\left( \deg \right)\end{matrix}}$ 2X (mi) 6000 21.37 45 8 66..4 5605 3000 29.3 30 18 59.33963 1000 42.1 15 72 57.1 2051 500 52.86 11 162 63.86 1512 250 59.726.923 338 66.6 955.4 124.27 59.57 3.214 1568 62.8 444.4

Values of θ_(n) less than 68° are used herein, which is the approximatevalue for a GS satellite for a latitude of 60°, in order to limit theresolution degradation such as that which occurs for GS satellitemeasurements at high latitude and to approximately cancel theatmospheric effects at the edge of the scan for ratio measurements. Theresidual atmospheric effects give a worst case result for themeasurements at altitude z compared with those at GS altitude.

Table 3 is an intermediate step calculational tool for the scalingfactors (D_(g)/D_(z))^(n′) to allow the performance to be adjusted forthe case of equal system cost for n satellites at altitude z and 5satellites at GS altitude where the cost varies as the cube of thecollector diameter, see Eq. (55). For example, for the case of 72satellites at 1000 mi altitude, the ratio of the number of satellites ataltitude z and GS altitude, n/5, is 14.4. Then, for a cost which variesas D³, the diameter of the collector at altitude z would need to bereduced by about 2.433 times to allow a comparison at the same cost. Thelast two columns in this table give the performance reduction for equalcost where the performance varies as D (the last column) or D² (next tolast column). Five satellites for coverage from GS orbit are selectedsince, as previously discussed, 3 satellites are required for partialearth coverage up to 60° latitude from this orbit. It is thenoptimistically assumed that full earth coverage could be obtained withtwo additional high altitude satellites for a total of five satellites.This same method of cost analysis could be used for other problems withdifferent numbers of satellites, different altitudes and different costmodels.

TABLE 3 Intermediate Table of Scaling Factors (D_(g)/D_(z))^(n′) forCalculating the Performance for Systems of Equal Cost for n Satellitesat Altitude Z and 5 Satellites at Geosynchronous Altitude g for a CostProportional to D³. z (mi) n n/5 (D_(g)/D_(z))² D_(g)/D_(z) 6000 8 1.61.368 1.170 3000 18 3.6 2.349 1.533 1000 72 14.4 5.919 2.433 500 16232.4 10.163 3.188 250 338 67.6 16.59 4.074 124.27 1568 313.6 46.16 6.794

Table 4 gives a performance comparison for measurements at altitude zand at GS altitude g for the signal S for the case of shot noise; andfor the signal to noise ratio S/N and the square of the resolution d²for the case of shot noise or the case of noise independent of signalwhich give the same result for systems with the same F number as wasdiscussed previously following Eq. (43). The results are given for asystem with n satellites at altitude z and 5 satellites at GS altitudefor full earth coverage in the frame time T with the same sensor system.That is, the systems have the same diameter collector and the samedetector capability. These results are not adjusted for equal cost. Asshown, there is a very large performance advantage for all performancemeasures for a low altitude satellite system as compared to a GS system,but at a much higher cost. For example, a system at 250 mi altitude withof the order of 338 satellites has a signal advantage of the order of90,000 times for measurement at the edge of the scan, ED, or 1 milliontimes along the nadir, NA. For this same altitude, the square of theresolution for measurements at the edge of the scan for an LMAS systemis of the order of 300 times higher than for measurements for a GSsatellite system and of the order of 1,000 times higher, for this samecase, for measurements on the nadir. The calculations at the edge of thescan are at the angles given in Table 2.

TABLE 4 Performance Comparison for Measurements for a System of nSatellites at Altitude z and 5 Satellites at Geosynchronous Altitude gfor the Signal (S), signal to noise (S/N) and Square of the Resolution(d²) for Measurements on the Nadir NA and at the Edge of the Scan ED forShot Noise and Noise Independent of the Signal. z (mi) n NA or ED$\frac{{S_{z}}^{+}}{S_{g}}$$\left( \frac{d_{g}}{d_{z}} \right)^{2}\mspace{14mu} {or}\mspace{14mu} \frac{\left( {S\text{/}N} \right)_{z}}{\left( {S\text{/}N} \right)_{g}}$6000 8 NA   27 5.2 ED   19 4.4 3000 18 NA  230 15 ED  350 19 1000 72 NA 9000 95 ED 11000 100 500 162 NA 9.1*10⁴ 300 ED 2.0*10⁴ 140 250 338 NA1.1*10⁶ 1100 ED 8.9*10⁴ 300 124.27 1568 NA 1.9*10⁷ 4300 ED 3.3*10⁶ 1800⁺For Shot Noise Only

Table 5 gives the performance comparison of Table 4 for the case ofequal system cost for an LMAS system at altitude z withcontiguous/overlapping full earth coverage and a GS system at altitude gwith partial earth coverage, where the cost is proportional to D³. Thescaling factors of Table 3 are used to reduce the performance given inTable 4 to obtain the results of Table 5 for equal cost, as given by Eq.(55) and discussed for Table 3. As shown in Table 5, at lower altitudesthe performance of the measurements at altitude z is substantiallyhigher compared to that at GS altitude. That is, the lower the satellitealtitude, the higher the performance compared to a GS system. Forexample, for measurements at the edge of the scan, the signal for asystem at 6,000 mi altitude is of the order of 14 times higher than thatof a GS system whereas for a system at 124.3 miles the signal is of theorder of 70,000 times higher. Also, at the edge of the scan, the squareof the resolution is of the order of 4 times higher for a system at6,000mi compared to a GS system whereas it is of the order of 270 timeshigher for a system at 124.3 mi altitude compared to a GS system.Moreover, the performance comparison discussed here and shown in Table 5is for systems for the same cost.

TABLE 5 Performance Comparison of Table 4 Adjusted for Equal System Costfor Cost Proportional to D³ (for Measurements for a System of nSatellites at Altitude z and 5 Satellites at Geosynchronous Altitude gfor the Signal (S), signal to noise (S/N) and Square of the Resolution(d²) for Measurements on the Nadir NA and at the Edge of the Scan ED). z(mi) n NA or ED $\frac{{S_{z}}^{+}}{S_{g}}$$\left( \frac{d_{g}}{d_{z}} \right)^{2}\mspace{14mu} {or}\mspace{14mu} \frac{\left( {S\text{/}N} \right)_{z}}{\left( {S\text{/}N} \right)_{g}}$6000 8 NA  20 4.4 ED  14 3.7 3000 18 NA  97 9.9 ED  150 12 1000 72 NA1500 39 ED 1800 43 500 162 NA 9000 95 ED 2000 44 250 338 NA 6.9*10⁴ 260ED 5.4*10³ 73 124.27 1568 NA 4.1*10⁵ 640 ED 7.2*10⁴ 270 ⁺For Shot NoiseOnly

The method of Table 5 may also be used to make equal cost comparisons ofthe performance for satellite systems at different altitudes by takingthe ratio of the performance of those systems at those altitudesassuming the same value of θ_(n) at the two altitudes. For example, forthe case of signal measurements on the nadir, the performance of asystem at 124.27 mi altitude is of the order of 2.0×10⁴ times higherthan that of a system at 6,000 mi with the same cost, i.e., the ratio of4.1*10⁵/20. We note that the system at 6,000 mi altitude would use 8satellites each carrying a large sensor and the system at 124.27 miwould use 1,568 satellites, each carrying a very small sensor.

The method of Eq. (55) and Table 5 may also be used to determine theperformance of systems with a small fraction of the cost of a GSsatellite system. For example, for a cost of ⅛, one-eighth, that of a GSsystem, the diameter of the LMAS system would be reduced by a factor of2 for a cost which varies as D³. The performance is then found from Eq.(23) for the signal ratio which decreases by a factor of 4 for thischange and from Eqs. (30) and (31) for the signal to noise ratio and thesquare of the resolution which decrease by a factor of 2 for thischange. As a result, the corresponding performance of all the systemsshown in Table 5 would still be higher than that of the GS system, andthe performance of the lower altitude systems would be higher than thatof a GS system by very large factors.

For the equal cost comparisons of Table 5, the performance of an LMASsystem with n satellites at altitude z can also be compared with that ofa GS system with only 1 satellite, which would give less than ⅓,one-third, full earth coverage and which would obviously give an unfairadvantage to the GS satellite system in the performance comparison. Inthis case, the diameter of the telescope for the LMAS configurationwould be reduced by a factor of the order of 1.71, the cube root of 5,see Eq. (55), beyond that given in Table 3 and already included in theequal cost comparison of Table 5. As can be determined from Table 5 andEqs. (23) and (31), the performance comparison for a complete LMASsystem at altitude z versus only 1 GS satellite still yields very largeperformance advantages for the LMAS system. For example, for an altitudeof 124.3 mi (200 km) and for measurements on the edge of the scan, anLMAS system would have a factor of the order of 25,000 times higherperformance for the signal or a factor of the order of 160 times higherperformance for the square of the resolution as compared to a GS systemwith only 1 satellite, equal cost, and only very partial, less than ⅓,earth coverage.

The method of Table 3 may be applied to calculate the performance forthe case of equal cost for the orbital configuration of q low altitude,high performance satellites (which can be used to obtain improvedcoverage and greatly reduced revisit time) and for the case of a singlelow altitude, high performance satellite, see FIG. 14 and the priordiscussion of this case. For coverage using 1 set of 4 satellites vs. 1satellite for an approximate 200 km altitude orbit, this would giverevisit times for observations anywhere on the earth of once every 8 or9 orbits. For the case of a cost model which varies as the cube of thetelescope diameter as discussed for Table 3, measurements could be madewith this small constellation of 4 satellites with telescopes with areduced diameter of about 1.59 times at the same cost as a highperformance system. This would result in about 26% less spatialresolution for the case where the resolution is signal limited, see Eq.(31), or a factor of 1.59 less resolution for the case where theresolution is diffraction limited. If this system of 4 satellites iscompared to a 2 satellite system, the difference in resolution wouldonly be about 12% for the signal limited case or a factor of 1.26 forthe diffraction limited case. This represents only one of a large numberof possible methods of use.

Table 6 shows the cost advantage for a low or mid-altitude satellite,LMAS, system versus a GS system for the GS system to obtain the sameperformance as the LMAS system, as shown in Table 5. This is for a costwhich varies as D³. As shown in Table 6, the cost advantage of the LMASsystem versus a GS system varies from of the order of 50 times to of theorder of 250 million times depending on altitude. Thus, to obtain thesame performance as a given LMAS system, the cost of a GS system wouldhave to be increased by the factors shown in Table 6, for example, about20 million times for a low altitude system at 124 mi at the edge of thescan. These very large cost advantages can be understood as follows. Inthis case, the term (d_(g)/d_(z))², the ratio of the resolution squaredat GS altitude and at altitude z, varies as the ratio of the collectordiameters, see Eq. (31). Then, to account for only a factor of 2 changein (d_(g)/d_(z))², it requires an increase in cost of 8 times since weassume the cost varies as D³. Similarly, for a factor of 73 change in(d_(g)/d_(z))² as shown in Table 5 at the edge of the scan for 250 mialtitude, the increased cost of a GS system to obtain this performanceis (73)³ or a factor of the order of 400,000 times. Thus, there is avery large increase in cost to obtain improved performance and theperformance improvements shown for low- and mid-altitude systems arevery large. Other cost models using, for example, a cost which varies asD^(a) could also just as easily be used with this method.

TABLE 6 Ratio of the Cost of a Geosynchronous (GS) System to the Cost ofa System at Altitude Z for the GS System to Obtain the Same Performanceas the Satellite System at Altitude Z for a Cost Proportional to D³ (forMeasurements on the Nadir NA and at the Edge of the Scan ED) COST GSSYSTEM/COST Z (mi) NA or ED ALT Z SYSTEM 6000 NA 87 ED 53 3000 NA 960 ED1800 1000 NA 5.9 * 10⁴ ED 7.7 * 10⁴ 500 NA 8.5 * 10⁵ ED 8.7 * 10⁴ 250 NA1.8 * 10⁷ ED 3.9 * 10⁵ 124.27 NA 2.6 * 10⁸ ED 1.9 * 10⁷

The improved performance given in table 5 is based on the strongdependence of resolution on altitude, see Eq. (31), and the use of costand constellation models to improve the system performance as given intables 3-6 and the associated discussion. More generally, Eq. (31) alsohas a strong dependence on the angle θ_(n) through the slant range R andon A which varies as cos²θ_(n). The performance can then be improvedwith respect to the angle θ_(n) in the same way it was for altitude.

Table 7 gives, as an example, the square of the resolution for selectedangles, adjusted for equal cost, for a system of n satellites ataltitudes of 500 and 124.3 mi compared to that of 5 satellites atgeosynchronous altitude g. As shown in table 7, for large angles θ_(n)of about 63°, the performance at the edge of the scan is significantlypoorer than that at the nadir. The resolution on the edge of the scan isgreatly improved, however, by reducing θ_(n). In particular, it can beimproved by approximately 3 times by reducing θ_(n) from about 63° to51°, and by more than 10 times by reducing θ_(n) from 63° to 30°. Thus,substantially higher levels of performance can be obtained on the edgeof the scan, for the same cost, using smaller angles θ_(n). Further, theresolution on the edge of the scan is the limiting factor in theperformance since the resolution falls off sharply with increasing angleθ_(n), see Eq. (31). In addition, the resolution also falls off sharplyfor large angles θ_(n) due to atmospheric transmission loss.

TABLE 7 Square of the Resolution Ratio vs. Angle adjusted for EqualSystem Cost for n satellites at altitudes z of 500 and 124.27 mi and 5satellites at Geosynchronous altitude g- θ_(n) (see Eq. (6A)), θ′ is thescan angle seen from the center of the earth, d² the square of theresolution at GS altitude g or altitude z, NA the nadir, and ED the edgeof the scan. θ_(n) (deg) θ′/2 (deg) n${\left( \frac{d_{g}}{d_{z}} \right)^{2}}_{NA}$${\left( \frac{d_{g}}{d_{z}} \right)^{2}}_{ED}$ z = 500 mi 63.86 11 16295 44 51.6 7.5 288 94 140 30.41 3.7 1250 95 470 15.73 1.8 5000 115 860 z= 124.27 62.79 3.21 1568 640 270 51.5 2.14 3700 580 790 29.98 1.0 16200600 3000

As previously discussed for active measurements, Eq. (50) gives theratio of the measured signals for altitude z and GS altitude where thesatellites at the two altitudes have the same average output power, Eq.(48), for measurement. For measurements with the same resolution,d_(z)=d_(g), the equation for the signal ratio (or the ratio of thesignal to noise terms) for an active system at altitudes z and g isequal to A*/A (or √{square root over (A*/A)}) times that of a passivesystem. Then a performance comparison for the signal ratio (or the ratioof the signal to noise terms) for an active system is equal to A*/A (or√{square root over (A*/A)}) times that of the results for a passivesystem, given by tables 4 and 5. Table 5 is adjusted for equal systemcost and is for the case of an active system where the cost of thesatellite power source is a small part of the total system cost. Thesetables show very large performance advantages for low or mid-altitudeactive systems as compared to an active system at GS altitude. Forexample, for measurements of the signal at 250 miles altitude on thenadir, the performance advantage of an active LMAS system versus anactive GS system is of the order of 1 million times from Table 4 and70,000 times from Table 5.

However, for the case of an active system where the signal to noiseratio at altitudes z and g are the same, Eq. (51) shows the resolutionimprovement for an active system for any 2 altitudes g and z is equal to√{square root over (A*/A)} times the square of the resolutionimprovement for a passive system. Then, the resolution improvementd_(g)/d_(z) for an active system is given by √{square root over (A*/A)}times the columns for (did, in tables 4 and 5. For example, formeasurements at 250 mi altitude on the nadir, the resolution improvementis about 1,000 times from table 4 and 250 times from table 5 which ismuch larger than the corresponding improvement for passive systems.

The methods of this disclosure can also be used to obtain improvedsatellite revisit time for low altitude satellites. The important termsin Eq. (59) which can be used to improve the revisit time are(R_(g)/R_(z))² which depends on altitude and θ, A which depends oncos²θ_(n), θ, θ′, and d². The dependence on altitude will be treatedfirst. For altitudes z much less than g, R_(g)/R_(z) will be large andD_(g)/D_(z) will be much greater than one. This is the condition neededto obtain much improved satellite revisit time as discussed previouslyin conjunction with Eqs. (59)-(61). For a low altitude satellite with analtitude g of 500 mi, altitudes z of 200 mi, 124.3 mi (=200 km), and 90mi are used as examples. Table 8 shows the parameters θ/2, θ′/2, θ_(n)and Y for these altitudes for a value of θ_(n) of approximately 63°.Large angles such as this, or larger, are typically used for satellitesystems to obtain as much coverage as possible for a single satellitepass. For an altitude of 500 mi, the value used for θ_(n) isapproximately 68° ′ This value is required to provide coverage for therotation of the earth, θ′=25.22°, in the time of one satellite orbit. Ifthis condition is not met, much poorer revisit times at 500 mi altitudewill result.

TABLE 8 Parameters for low altitude systems at various altitudes z - θis the scan angle of the satellite, θ′ is the scan angle as seen fromthe center of the earth, and Y is the chord length in the along trackdirection. Z θ/2 θ′/2 (θ + θ′)/2 Y (mi) (deg) (deg) (deg) (mi) 500 55.4612.61 68.07 865 200 58.82 5.17 63.99 357 124.3 59.57 3.21 62.78 222 9059.90 2.33 62.25 161

Table 9 gives the revisit time T_(R), the ratio of the telescopediameters D₅₀₀/D_(z) at 500 mi and altitude z, and the number ofsatellites n at altitude z which can be obtained for the same cost andthe same resolution as one satellite at an altitude of 500 mi for signallimited systems. The results are shown for measurements in the nadirdirection, NA, and also on the edge of the scan, ED, at an angle θ/2. Asshown, the revisit time can be improved by more than 100 times fromapproximately 12 hours, 720 minutes, for a single satellite at 500 mialtitude to approximately 7 minutes at 124 mi altitude and to almost 2minutes at 90 mi altitude. In addition, semi-continuous coverage can beobtained on the edge of the scan at 90 mi altitude. These results arefor the same cost, the same resolution, and the same signal to noise asa single satellite at 500 mi altitude for a cost which varies as D³.

TABLE 9 Revisit time T_(R) for signal limited systems at variousaltitudes z with the same cost and resolution - D is the telescopediameter, n the number of satellites, NA the nadir, and ED the edge ofthe scan. Z T_(R) (mi) NA, ED D₅₀₀/D_(z) n (min) 500 NA 1 1 720* ED 1 1720* 200 NA 3.58 46 45 ED 5.00 125 15 124.3 NA 7.12 361   7.4 ED 10.961320   1.9 90 NA 11.42 1488   2.3 ED 18.30 3042 Cont.^(†) ^(†)Continuouscoverage with, in addition, a significant improvement in the square ofthe resolution. *Approximate value

The revisit time can also be reduced (further) to providesemi-continuous coverage for a low altitude satellite. The number ofsatellites needed for continuous coverage at altitude z is given ingeneral by Eq. (6) and for the case of symmetrical constellations byn=2n_(e) ², see Eq. (7C). Continuous coverage can be obtained with Eqs.(7C), (59) and (60), by one—decreasing the altitude as discussedpreviously to obtain n_(z) satellites at altitude z for the sameresolution at altitudes z and g; two—decreasing the resolution of themeasurements at altitude z, relative to those at altitude g, by thefactor

$\begin{matrix}{\frac{d_{z}}{d_{g}} = \left( \frac{2\; n_{e}^{2}}{n_{z}} \right)^{\frac{1}{6}}} & (65)\end{matrix}$

for a signal limited system. Table 10 gives results for the reduction inresolution needed to obtain continuous coverage on the nadir and at theedge of the scan. As shown, for measurements on the nadir, thiscorresponds to a reduction in resolution of approximately 1.28 times at124.3 mi altitude and 1.13 times at 90 mi altitude.

TABLE 10 Resolution ratio d_(z)/d_(g) at altitude z and 500 mi forsignal limited systems, sl, for semi-continuous earth coverage formeasurements on the nadir, NA, and the edge of the scan, ED, at variousaltitudes. NA, Z ED (d_(z)/d_(g))_(sl) 500 NA 2.25 ED 2.25 200 NA 1.55ED 1.32 124.3 NA 1.28 ED 1.03 90 NA 1.13 ED 1.0

Table 11 shows the effect on the revisit time of reducing the angleθ_(n) for measurements with n satellites at altitude z at the edge ofthe scan with the same cost and resolution as 1 satellite at 500 mialtitude. The results given here use the angular dependence of theperformance in Eqs. (58) and (59) to improve the revisit time. As shown,a small change in the angle θ_(n) from approximately 63° to 52° issufficient to produce semi-continuous coverage for altitudes of both 200mi and 124.3 mi. The increase in performance due to this change in anglealso produces, in addition, a substantial improvement in the square ofthe resolution at 200 mi altitude as well as at an altitude of 124.3 mi.

TABLE 11 Revisit time T_(R) vs. Angle for Signal Limited Systems at theedge of the scan with the same cost and resolution - z is the altitude,θ_(n) see Eq. (6A), θ′ is the scan angle seen from the center of theearth, D the telescope diameter, and n the number of satellites. Z θ_(n)θ′/2 T_(R) (mi) (deg) (deg) D₅₀₀/D_(z) n (min) 500 68.07 12.61 1 1 720*200 63.99 5.17 5.00 125 15 52.64 3.47 16.28 1352 cont.^(†) 124.3 62.783.21 10.96 1320   1.9 51.5 2.14 34.92 3698 cont.^(††)^(†),^(††)Continuous coverage with, in addition, a substantialimprovement in the square of the resolution. *Approximate value

The improved performance obtained by using lower altitudes z and smallerangles θ_(n) than at altitude g could alternately be used to improve thespatial resolution of the measurements. For signal limited performance,it follows from Eqs. (55), (58) and (59) that if the spatial resolutionis improved by the maximum amount while maintaining the same or betterrevisit time as that at altitude g, then the improvement in resolutionis

$\begin{matrix}{\left( \frac{d_{g}}{d_{z}} \right)^{2} = \frac{D_{g}/D_{z}}{\left( {\theta_{g}^{\prime}/\theta_{z}^{\prime}} \right)^{\frac{1}{3}}}} & \left( {65A} \right)\end{matrix}$

where θ_(g)/θ′_(z) is the number of satellites needed to maintain thesame angular coverage as at altitude g and its value is rounded up tothe next largest integer. The term D_(g)/D_(z) in Eq. (65A) is ford_(z)=d_(g) and the values given in tables 9 and 11 produce largeresolution improvements at the same cost if used for this purpose.

The effects of diffraction limited performance are now considered. Table12 compares the ratio of the performance of diffraction limited systemson the nadir at GS altitude with those at various altitudes z, see Eq.(64). It also compares the performance of signal limited systems withthis diffraction limited performance. This is for a system of nsatellites at altitude z which give semi-continuous, contiguous coverageof the earth for the same cost, see tables 3 and 5. As shown, adiffraction limited system shows a greater improvement in resolution forall altitudes than a signal limited system. For example, the improvementin resolution for a diffraction limited system at 500 mi altitude isapproximately 14 times compared to a GS system whereas it is 9.7 timesfor a signal limited system. Thus, the gains in performance for adiffraction limited system are greater than the gains for a signallimited system given in tables 5 and 6. It may also be noted that forequal cost, the performance of all systems improves at lower altitudes.

TABLE 12 Performance comparison of Table 5 (for a system of n satellitesat altitude z and 5 satellites at Geosynchronous altitude g) for signallimited systems sl and diffraction limited systems d on the nadir forthe same cost. z (mi) (d_(g)/d_(z))_(sl) (d_(g)/d_(z))_(d) 6000 2.1 3.23000 3.1 4.8 1000 6.2 9.1 500 9.7 14 250 16 22 124.27 25 26

The case of low altitude diffraction limited systems is now considered.The resolution of diffraction limited systems is given by Eq. (63) andthe ratio of the resolution at altitudes g and z is given by Eq. (64).Then for the same resolution for altitudes g and z, it follows from Eq.(64) that

D _(g) /D _(z)=(R _(g) /R _(z))√{square root over (A)}  (66)

This limits the value of D_(g)/D_(z). Table 13 gives the value ofD_(g)/D_(z), the number of satellites, and the resulting revisit timefor diffraction limited systems with the same cost and resolution. Asshown, reducing the altitude and the telescope diameter significantlyimproves the revisit time, e.g., from 720 min at 500 mi altitude to 11min on the edge of the scan at 90 mi altitude. In addition, the revisittime for measurements on the edge of the scan can be significantlyfurther improved (reduced) by reducing θ_(n) as was used to improve therevisit time for signal limited systems, see Table 11.

TABLE 13 Revisit time T_(R) for diffraction limited systems with thesame cost and resolution at various altitudes z - D is the telescopediameter, n is the number of satellites, g is 500 mi, NA the nadir andED the edge of the scan. Z T_(R) (mi) NA/ED D_(g)/D_(z) n (min) 500 NA 11 720* ED 1 1 720* 190.8 NA 2.62 18 90 ED 3.04 28 90 124.3 NA 4.02 65 44ED 4.99 124 22 90 NA 5.56 171 22 ED 7.04 349 11 *Approximate value

For diffraction limited performance, semi-continuous coverage can beobtained from Eqs. (7C), (60), and (64) for

$\begin{matrix}{\frac{d_{z}}{d_{g}} = \left( \frac{2\; n_{e}^{2}}{n_{z}} \right)^{\frac{1}{3}}} & (67)\end{matrix}$

where n_(z) is the number of satellites for the same resolution ataltitudes z and g. (See table 13 for values.) Table 14 shows theresolution ratio d_(z)/d_(g) needed to obtain continuous coverage on thenadir at various altitudes for diffraction limited systems. As shown,the loss in resolution to achieve continuous coverage is about 2.9 or2.6 times at altitudes of 124.3 or 90 mi whereas for signal limitedsystems the loss in resolution is about 1.28 or 1.13 times for the samealtitudes, see table 10.

TABLE 14 Resolution ratio d_(z)/d_(g) at altitude z and 500 mi fordiffraction limited systems d for semi-continuous earth coverage formeasurements on the nadir. z (mi) (d_(z)/d_(g))_(d) 200 3.46 190.8 3.30124.27 2.89 90 2.61

The improved performance obtained by using lower altitudes z and smallerangles θ_(n) than at altitude g could also be used to improve thespatial resolution of diffraction limited measurements in the same wayas it was for signal limited measurements, see Eq. (65A). It followsfrom Eqs. (55) and (64) that if the spatial resolution is improved whilemaintaining the same or better revisit time as that at altitude g, thenthe improvement in resolution is

$\begin{matrix}{\left( \frac{d_{g}}{d_{z}} \right) = \frac{D_{g}/D_{z}}{\left( {\theta_{g}^{\prime}/\theta_{z}^{\prime}} \right)^{\frac{1}{3}}}} & \left( {67A} \right)\end{matrix}$

where θ′_(g)/θ′_(z) is the number of satellites needed to maintain thesame angular coverage as at altitude g and its value is rounded up tothe next largest integer. The term D_(g)/D_(z) in Eq. (67A) is ford_(z)=d_(g) and the values given in table 13 produce large resolutionimprovements at the same cost if used to improve the resolution insteadof the revisit time.

An ultrahigh performance system, as discussed earlier, is used to scan asmall area of interest that corresponds to θ″_(x) by θ″_(y), muchsmaller than the coverage area θ_(x) by θ_(y) used for each of the n lowor medium altitude satellites, see FIG. 20.

The advantage to using an ultrahigh performance system for a signallimited system is found from Eqs. (11), (12), (18), and (22) withθ_(x)=θ_(y) which shows the signal to noise ratio is given as

${S/N} \propto {{D\left( \frac{d}{R} \right)}^{2}\frac{1}{\theta}\left( {pq}^{\prime} \right)^{\frac{1}{2}}\cos^{2}\theta_{n}}$

The improvement in performance for an ultrahigh performance systemP_(ul) as compared to a standard system P where the field of view of thedetector array ≦θ″_(x) by θ″_(y) and for the same array size, range,frame time, telescope diameter, zenith angle and atmospheric path isgiven as

$\begin{matrix}{\frac{P_{ul}}{P} = \frac{\theta}{\theta_{ul}}} & (68)\end{matrix}$

where the performance P is given in terms of the signal to noise forsystems with the same resolution as

P=S/N   (69)

or in teems of the square of the resolution for systems with the samesignal to noise as

P=1/d ²   (70)

where the subscript yl applies for an ultrahigh performance system. Eq.(69) applies generally, whereas Eq. (70) applies assuming signal effectslimit the resolution and not the diffraction limit. Then for a targetarea with an approximate size of 50 miles by 50 miles and the parametersof Table 2, the gain in the performance, Eq. (68), is of the order of 90times at 6000 miles altitude, 60 times at 3000 miles, 30 times at 1000miles, 15 times at 500 miles, 10 times at 250 miles, and 5 times at 124miles. For a comparison of an ultra-high performance LMAS system ataltitude z and a standard GS system, these gains are in addition to thegains in performance shown in Table 5 for systems of equal cost ataltitude z and GS altitude. These performance improvements can be usedfor improvements in signal to noise, resolution (subject to thediffraction limit), or frame time. This capability can be used for oneor more up to all n satellites in the LMAS system.

One important result of using an LMAS constellation with a relativelylarge number of satellites is that the constellation is much lessaffected by the failure or loss of a single satellite. For example, theloss of one GS satellite providing coverage of North and South America,out of a system of 3 GS satellites, would be catastrophic, e.g., a lossof hurricane tracking and coverage in North America. On the other hand,the loss of one out of 338 satellites in an LMAS constellation at 250 mialtitude would only result in a small, 0.3%, loss of data. Also, theeconomic consequences of such a loss in terms of the replacement cost ofthe satellite would also be small for an LMAS constellation, a cost ofabout 1% of the cost of replacing one GS satellite.

An important advantage to using an LMAS constellation with much smallerdiameter instruments than a GS system, see Table 3, is that the LMASsystem is a relatively low technology system. As a result, thesatellites are not only much lower cost but they are also much easier todesign, build, launch, and they are less susceptible to failure due tomisalignment. A high performance, low altitude LMAS constellation wouldalso use much smaller diameter telescopes than a current highperformance, low altitude system. As a result, the LMAS system uses alower level technology and has the same advantages over a current highperformance, low altitude system as those described above, i.e., thesatellites are smaller, easier to design, build, and less susceptible tofailure.

There are various additional applications that can be found in astraight forward manner using the general methods given here. Forexample, for a pulsed or a continuous wave (CW) active (laser) systemreflecting off a hard target with reflectivity r, ΔR in Eq. (44) isreplaced by r, and Eqs. (45) and (47-51) are the same as before withA*=1, and the same analysis applies. Also, if instead of requiring thatEq. (48) holds for active systems, it could be required that the energyoutput from the whole set of n_(z) satellites at altitude z be equal tothat of the whole set of n_(g) satellites at altitude g in a given frametime. That is, n_(z) times the left side of Eq. (48) equals n_(g) timesthe right side of Eq. (48). The signal ratio in Eq. (50) is then reducedby the factor n_(z)/n_(g) and as a result the ratio of the resolutionsfor an active system, Eq. (51), (as well as the ratio of the (S/N) ataltitudes z and g) is reduced by the factor (n_(z)/n_(g))^(1/2). Thewhole analysis then proceeds as before. The methods given here are alsoapplicable to detector arrays of different size. If a detector array has(pq′)_(z) elements at altitude (configuration) z and (pq′)_(g) elementsat altitude (configuration) g, then it follows from Eq. (18) that forsignal limited systems the signal ratio, Eq. (23), is proportional toB=(pq′)_(z)/(pq′)_(g) and the ratio of the signal to noise terms, Eq.(29) and the ratio of the squares of the resolution, Eq. (31), isproportional to the square root of B. This also applies to low altitudesatellite systems. That is, Eqs. (31A), (58), and (59) are alsoproportional to the square root of B. As a result, the performance ofsatellite systems can be improved by increasing the number of resolutionelements in a detector array.

For a spherical earth, the designation of a particular great circle onthe earth as being an equatorial plane is arbitrary except with respectto the rotation of the earth. Then the methods of constellation designgiven herein apply to any great circle of the earth. That is, themethods apply to the entire constellation of satellites and orbits whichis rotated by a polar angle which can vary from 0° to 180° with respectto the polar axis.

For the case of the q satellites of paragraph 94, the number of orbitsfor full earth coverage or coverage over a portion of the earth can bereduced, to as little as one orbit, using approximately equally spacedmultiple sets of q satellites.

The k-fold overlapping stereo coverage of paragraph 83 gives the 3dimensional position of each point on the earth and in the surroundingatmosphere and space over the earth at essentially each point in time.Then the 3 dimensional track of any given object, e.g., a missile, isobtained by following the location of that object as a function of time.The velocity of that object is given approximately as the change in theposition of the object divided by the change in time.

It is noted that the signal (signal to noise) limited performancesections beginning with paragraph 112 and paragraph 125 also apply todiffraction limited systems, e.g., the calculation of signal to noiseratio, with the exception of the diffraction limited resolution which istreated separately. Also, the signal (signal to noise) limitedperformance section beginning with paragraph 112 also contains the caseof noise independent of signal, beginning with paragraph 117, whichgives essentially the same result as signal (signal to noise) limitedperformance as discussed in paragraph 118.

While only selected embodiments have been chosen to illustrate thepresent invention, it will be apparent to those skilled in the art fromthis disclosure that various changes and modifications can be madeherein without departing from the scope of the invention as defined inthe appended claims. For example, the size, shape, location ororientation of the various components can be changed as needed and/ordesired. Components can have intermediate structures disposed betweenthem. The functions of one element can be performed by two, and viceversa. The structures and functions of one embodiment can be adopted inanother embodiment. It is not necessary for all advantages to be presentin a particular embodiment at the same time. Every feature which isunique from the prior art, alone or in combination with other features,also should be considered a separate description of further inventionsby the applicant, including the structural and/or functional conceptsembodied by such features. Thus, the foregoing descriptions of theembodiments according to the present invention are provided forillustration only, and not for the purpose of limiting the invention asdefined by the appended claims and their equivalents.

1. A method of performance analysis in satellite constellation designcomprising: comparing the performance of satellites with two differentconfigurations as a ratio of the spatial resolution of the twomeasurements for a diffraction limited system, as a ratio of the spatialresolution of the two measurements for the same signal to noise ratiofor a signal limited system, or as a ratio of the signal to noise of twomeasurements for the same spatial resolution for a signal limited or adiffraction limited system.
 2. The method of claim 1, wherein the twodifferent configurations include two different altitudes.
 3. The methodof claim 1, wherein the spatial resolution is projected on earth'ssurface.
 4. The method of claim 1, wherein the spatial resolution hasparameters that include a diameter of a collector, a distance from thesatellite to a measurement area, and a zenith angle of the measurement.5. The method of claim 1, wherein the satellites instruments are signallimited systems, or diffraction limited systems, where a signal limitedsystem is equivalent to a signal to noise limited system and givesessentially the same result as a noise independent of signal system ifthe noise independent of signal system uses the same number formeasurements with the two systems.
 6. The method of claim 5, wherein thesignal limited system is a signal limited geosynchronous replacementsystem, or a signal limited low altitude replacement system.
 7. Themethod of claim 6, wherein the diffraction limited system is adiffraction limited geosynchronous replacement system or a diffractionlimited low altitude replacement system.
 8. The method of claim 5,wherein the performance of a low or intermediate altitude satellite LMASsystems is compared to geosynchronous or other altitude satellitesystems assuming the frame time is the same.
 9. The method of claim 8,wherein the frame time is the time to cover a given coverage area onearth for a single satellite.
 10. The method of claim 6, furthercomprising comparing performance of a full constellation LMAS systemwith a number of satellites that provides contiguous/overlapping,continuous earth coverage with a geosynchronous or other altitudesatellite system that provides earth coverage for the same measurementtime.
 11. The method of claim 6, further comprising comparing theperformance of very low altitude LMAS system with current low altitudesatellites for a frame time that changes with altitude.
 12. The methodof claim 11, wherein the frame time is the time to cover a givencoverage area on earth for a single satellite.
 13. The method of claim11, wherein the low altitude LMAS system uses partial constellations ofsatellites.
 14. The method of claim 6, wherein a ratio of resolutions ofmeasurements from two different satellite systems at different altitudesand with different configurations depends on the collector diameter, thedistance from the satellite to a measurement area, and the zenith angleof the measurements for both signal and diffraction limited passivesystems.
 15. The method of claim 14, wherein the ratio further dependson one or more of the angular field of measurement and the frame time ofthe measurement for signal limited passive measurements.
 16. The methodof claim 14, wherein a ratio of signal to noise measurements from twodifferent satellite systems at different altitudes and with differentconfigurations depends on the collector diameter, the distance from thesatellite to the measurement area, the zenith angle of the measurements,the angular field of measurement and the frame time of measurement forsignal and/or diffraction limited passive measurements.
 17. The methodof claim 8, further comprising comparing the performance of pulsed andcontinuous wave active systems in terms of ratio of the signals, theratio of the signal to noise and the ratio of the spatial resolution forsystems at any two altitudes.
 18. The method of claim 8, wherein theperformance of signal limited passive systems and the signal to noise ofdiffraction limited passive systems improves with the number of spatialresolution elements in a detector array.
 19. A method of cost analysisfor satellite constellation design and satellite instrument designcomprising: comparing the costs of two different constellations ofsatellites as a ratio of functions which depend approximately on anumber of satellites in a constellation, and a collector diameter of thesatellite instrument; and adjusting the number of satellites, theorbital altitude of the constellations, and the collector diameter ofthe instrument based on the cost.
 20. The method of claim 19 wherein thecost of the constellation depends approximately on the number ofsatellites in the constellation and the collector diameter of thesatellite instruments.
 21. The method of claim 19, wherein the ratio ofthe costs of the constellations varies approximately as the ratio of thenumber of satellites in a constellation times the ratio of the cube ofthe collector diameters.
 22. The method of claim 19, wherein thecollector diameter and the altitude of the constellation is reduced andthe number of satellites is increased to maintain the same system cost.23. The method of claim 19, wherein the collector diameter is furtherreduced to obtain a lower system cost with the same increase in thenumber of satellites.
 24. The method of claim 19, further comprisingreducing the collector diameter at an altitude to adjust performance andcost.
 25. A method of performance/cost analysis, optimization for a lowor intermediate altitude satellite LMAS system to replace ageosynchronous system or a system at another altitude comprising:changing one or more parameters of a performance model to improveperformance which changes the system configuration; using aconstellation model to determine the number of satellites required;using a cost model to adjust the cost, a number of satellites, and acollector diameter as required which further changes the systemconfiguration; using a performance model to determine the change inperformance based on changes in the parameters in the performance, cost,and constellation models; and iterating the changes in configuration tooptimize the performance and cost or obtain a desired set of systemproperties.
 26. The method of claim 25, wherein the system propertiesinclude one or more of cost, performance and complexity of dataanalysis.
 27. The method of claim 25 further comprising reducing thealtitude of a satellite and/or a zenith angle to increase theperformance; using a constellation model to determine the number ofsatellites required to maintain contiguous/overlapping continuous earthcoverage; using a cost model to determine a collector diameter of theLMAS system relative to that of the GS altitude system to maintain thesame or the desired system cost with a larger number of satellites;using a performance model to compare the performance of the loweraltitude system with many satellites and a reduced collector diameter tothat of a GS altitude system with a few satellites with a largecollector diameter; and repeating the above steps at various altitudesand zenith angles to find an optimum or a desired set of performance andsystem properties.
 28. The method of claim 25, wherein the cost model isused to determine a smaller collector diameter for the lower altitudesystem if a lower cost system is required.
 29. The method of claim 25,wherein the performance includes resolution or signal to noise ratio.30. A method for optimizing a revisit time of very low altitude LMASsystem to replace a standard low altitude system comprising: changingone or more parameters of a performance model to improve performancewhich changes the system configuration using the performance model tocalculate an amount of reduction of a collector diameter for thestandard low altitude system to obtain an approximately same propertyfor the two systems; using a cost model to determine a number n_(c) ofsatellites in the very low altitude LMAS system that can be obtained atthe same cost as the standard low altitude system; using a partial orfull constellation model to determine a number n_(p) of equally spacedsatellites in each orbital plane that can be filled with the numbern_(c) of satellites; and providing a revisit time as a ratio of anorbital period to the number of satellites in each orbital plane, n_(p).31. The method of claim 30 wherein the altitude of the standard lowaltitude system is any altitude higher than that of the very lowaltitude LMAS system.
 32. The method of claim 30, wherein the propertyincludes resolution or signal to noise.
 33. The method of claim 30further comprising: reducing an altitude of the very low altitude LMASsystem relative to that of the standard system and/or decrease a zenithangle to increase performance of the low altitude system; using theperformance model to calculate an amount of reduction of a collectordiameter for the standard lower altitude system to obtain the sameresolution for the two systems; determining the number n_(e) ofsatellites to provide coverage over an equatorial plane and determiningthe number n_(p) of equally spaced satellites in each orbital plane thatcan be filled with the number n_(e) of satellites.
 34. The method ofclaim 30, further comprising reducing the revisit time to providesemi-continuous coverage for the very low altitude LMAS system at thesame cost as the standard low altitude system.
 35. The method of claim30, further comprising reducing the altitude; decreasing, degrading, theresolution of the measurements for the very low altitude system; andincreasing the number of satellites to provide full constellations whichhave semi-continuous coverage.
 36. The method of claim 30, furthercomprising reducing the altitude to improve the spatial resolution ofthe measurements at the same cost as for the standard low altitudesystem; and increasing the number of satellites and adjusting theresolution in the very low altitude system to maintain the same angularcoverage and approximately the same revisit time as that of the higheraltitude system but with a higher spatial resolution.